Syrian government forces have used chemical weapons in the country’s civil war, the Israeli military’s top intelligence analyst said on Tuesday, citing photographic evidence of people "foaming from the mouth."

If the claim by Brigadier-General Itai Brun is confirmed, it would mean Syria’s President Bashar Assad has crossed what the State Department has previously described as a red line that would trigger some form of U.S. response. President Barack Obama also warned Assad using chemical weapons would be a "tragic mistake" that would have "consequences."

Brun told a conference at the Institute of National Security in Tel Aviv that photographs of victims showing foam coming out of their mouths and contracted pupils were signs that a deadly gas had been used.

"One of the main characteristics of the recent events in Syria is the increasing use of ground-to-ground missiles, rockets and chemical weapons by the Syrian regime. There is a wide-range usage of missile, rockets and more by the Syrian weapons array," he said, according to a translated transcript of his remarks provided by the Israel Defense Forces.

"According to our professional assessment, the regime has used deadly chemical weapons against armed rebels on a number of occasions in the past few months," he said.

"For instance, on March 19, 2013, victims suffered from shrunken pupils, foaming from the mouth, and other symptoms which indicate the use of deadly chemical weapons. The type of chemical weapons was likely sarin, as well as neutralizing and non-lethal chemical weapons," he added.

According to the Centers for Disease Control, sarin, a nerve agent, causes symptoms including loss of consciousness, convulsions, paralysis, and respiratory failure that can be fatal.

George Ourfalian / Reuters file

Animal carcasses lie on the ground after what residents, Syrian rebels and Assad's regime all said was a chemical weapon attack in Khan al-Assal near the northern city of Aleppo, on March 23.

In March, Assad's regime and the rebels blamed each other for what both said was a chemical-weapon attack in Aleppo.

Responding to Brun’s comments, Pentagon Press Secretary George Little said in a written statement that the United States “continues to assess reports of chemical weapons use in Syria.”

“The use of such weapons would be entirely unacceptable. We reiterate in the strongest possible terms the obligations of the Syrian regime to safeguard its chemical weapons stockpiles, and not to use or transfer such weapons to terrorist groups like Hezbollah,” he added.

On Monday, Defense Secretary Chuck Hagel said the use of chemical weapons by Assad's forces would be a "game changer" and the United States and Israel "have options for all contingencies," Reuters reported.

Hagel met Prime Minister Benjamin Netanyahu in Jerusalem on Tuesday, the news service said, a day after flying in an Israeli military helicopter over the occupied Golan Heights on the edge of the fighting in Syria that has entered its third year.

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"This is a difficult and dangerous time, this is a time when friends and allies must remain close, closer than ever," Hagel, in remarks to reporters before his talks with Netanyahu, said about the United States and Israel.

Secretary of State John Kerry, who was in Belgium for a NATO meeting on Tuesday, that he did not have information that confirmed that the Syrians had used chemical weapons.

Earlier he said the alliance needed to consider its role in the crisis, Reuters reported. "We should also carefully and collectively consider how NATO is prepared to respond to protect its members from a Syrian threat, including any potential chemical weapons threat," he added.

Kerry said that the planning the alliance had already done was appropriate. 

Reuters contributed to this report.

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Danny Gold

Young male and female fighters in the Kurdish militia known as the Popular Protection Units line up in formation in front of supporters in Ras Al Ayn, Syria. Some see the border city as indicative of what could come if the Assad regime falls, where rebel groups with competing agendas attempt to fill the vacuum of power.

RAS AL AYN, Syria -- Yilmaz fears a visit to his cousins and friends on the other side of town will end with him assassinated by a sniper's bullet.

Syrian dictator Bashar Assad's forces were forced out of this divided border city months ago. Despite a tenuous cease-fire, the presence of different rebel groups who previously clashed and now coexist side by side has left many on the edge, fearing another breakout of war.

Yilmaz ran afoul of the Popular Protection Units (YPG), the country's most powerful Kurdish militia, when he became affiliated with the rebel Free Syrian Army (FSA). Some of his friends and relatives call him a traitor for siding with the FSA.

During the last phase of fighting, one of his cousins who, like him, had been an FSA activist, was killed by a YPG sniper. He knows the killer could be someone he spent his childhood with, or sees at family functions.

"Maybe my friends will kill me," the Kurdish former peace activist says. "Maybe one from them killed my cousin. It's complicated."

Some see this city as indicative of what could come if the Assad regime falls, where rebel groups with competing agendas attempt to fill the vacuum of power.

Danny Gold

A young rebel mans a checkpoint on a road leading to Ras Al Ayn, Syria.

Ras Al Ayn lies at the edge of Kurdish territory in the northeastern province of Hasakah near the border with Turkey. Though it is a majority Kurdish city, is it home to Christians, Chechens, Armenians and Arabs, and was once celebrated for its diversity and tolerance. It was one of the province's first city's to protest for the revolution.

Accounts differ on how the fighting started in Ras Al Ayn. In November, the FSA along with Islamist rebel groups like Jabhat Al Nusra and Ghuraba Al Sham attacked regime soldiers, eventually forcing them out. That coalition then clashed with the YPG.

A truce was eventually established, but quickly broken as the YPG and FSA again fought battles all over the city. After roughly two weeks of fighting, Syrian Christian dissident Michel Kilo arranged a cease-fire that has now held for nearly two months.

The city has still not recovered. Many residents fled during different phases of the fighting. While some semblance of normal life has returned, buildings still lay in ruins, many pockmarked with bullet holes. There is rarely electricity and water is scarce. Schools and hospitals have been ransacked and closed for months. Graffiti touting the different groups is spray-painted everywhere, and armed men from the rival factions are a constant presence.

Danny Gold

Weeks of fighting between rival factions have left homes and businesses in the Syrian city of Ras Al Ayn damaged, many beyond repair.

For some civilians caught in the crossfire, it's hard to draw a distinction. Alongside a road near where some of the most intense fighting took place, a group of Syrian-born Chechens relay stories of looting. "The Free Army and the YPG, they steal everything. We did not see freedom fighters, only thieves," said Tamer, a 47-year-old undertaker. "There isn't a difference between all these groups."

Outside a small cluster of shops near where a Syrian regime airstrike hit months ago, a butcher named Ahmed Shaabi laments that he has not been able to work for five days due to a lack of electricity. "This city has gone back a century," he said.

Rashid Abdullah, a construction worker sitting with Shaabi, thought the fighting in Ras Al Ayn played right into the hands of Assad. "It's wrong, it's all wrong. Our fight is with the regime," he said.

The Kurds make up roughly 10 percent of the Syrian population. The most powerful Kurdish political party, the Democratic Union Party (PYD), has sought to keep the war from encroaching in its territory, leading to a de facto truce of sorts with the regime.

This has drawn the ire of many rebel groups. They accuse the YPG, which is often seen as the political party's military wing, as being agents of the Assad regime. The YPG in turn accuses the FSA of being agents of Turkey and overrun with Islamists.

In Ras Al Ayn, this distrust resulted in months of conflict and nearly 300 deaths. 

Complicating things even further is the presence of a small brigade of Jabhat Al Nusra, the al Qaeda-affiliated Islamist group that is thought to be the most powerful jihadi faction. While JAN and the FSA often fight as allies, they've also clashed. The YPG claim to make no distinction between the Islamists and the FSA. 

Danny Gold

Photos of Kurdish rebels killed in battles line the walls at a house of martyrs in Ras Al Ayn, Syria.

On the YPG-controlled side of town in late March, a small crowd has come out for the opening of a house of martyrs, a community office dedicated to those killed in the fighting. Giant posters of slain fighters line the walls. Politicians and military commanders give speeches as a group of 20 young fighters, some who barely look out of high school, line up in formation clutching rifles and rocket-propelled grenades. When the ribbon is cut, the families of the deceased line up and greet the soldiers.

Not far away, a group of Free Syrian Army fighters known as the Mashaal Tammo brigade occupy a large house. One of the only mixed Arab and Kurdish fighting groups, the brigade is made up mostly of onetime peaceful protesters who, unlike the PYD, wanted to join the revolution early on. They eventually turned to the FSA.

"We took up arms and the reason was the PYD," says Marwan, a Kurd from Qamishli who fights with the brigade. "The PYD didn't give the people aid or anything, they pushed the people around."

The PYD has faced accusations of kidnapping and assassinating Kurds from opposing parties, including Kurdish activist and political leader Mashaal Tammo, for whom the brigade is named. Some Kurds see them as another authoritarian force trying to take control. They see the FSA as the only true proponents of the revolution.

Yilmaz is still focused on the revolution, but he's seen the toll it's taken on his city and grown weary. His only hope now is that the factions will focus their attacks on the regime.

"I hate what's happened in Ras Al Ayn," Yilmaz says. "The people, the civilians, so many of them have been killed. It's not the FSA's fault, it's not the YPG's fault, it's war. Just let peace stay in Ras Al Ayn."

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Related: 

• Is end in sight for one of world's longest-running conflicts?
• After decades of oppression, Kurds get 1st taste of freedom
• Full Syria coverage from NBC News

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AMMAN - Syrian forces and militiamen loyal to President Bashar al-Assad killed at least 85 people when they stormed a Damascus suburb after five days of fighting, opposition activists in the area said on Sunday.

There was no immediate confirmation of the activists' account of what they described as a "massacre," including of women and children, at Jdeidet al-Fadel. Syrian authorities have banned most independent media since the uprising began in 2011.

Syria's Sana state news agency said the military "inflicted big losses on terrorists in Jdeidet al-Fadel and destroyed weapons and ammunition and killed and wounded members of the terrorist groups."

Jamal al-Golani, a member of the Revolution Leadership Council opposition group, said the number of dead may be higher than 250 and that most of the victims were shot at close range, but the presence of army patrols made documenting all of them difficult.

"Jdeidet al-Fadel was militarily a lost cause from day one because it was surrounded by the army from every direction. There are almost no wounded because they were shot on the spot," he said.

NBC News has not confirmed the reports.

The killings happened over several days when pro-Assad forces stormed an area where there were up to 270 rebels, Golani said, adding that he had counted 98 bodies in the streets and 86 people who he said had been summarily executed in makeshift clinics where they were lying wounded.

Kerry: US to double nonlethal aid to Syrian opposition

The working class district, one of several Sunni Muslim towns surrounding the capital that have been at the forefront of the uprising against Assad, is situated near hilltop bases for elite loyalist forces, who mostly belong to Assad's minority Alawite sect, an offshoot of Shiite Islam that has dominated the country since the 1960s.

Abu Ahmad al-Rabi', an activist in the adjacent district of Jdeidet Artouz, said: "We documented 85 summarily executed, including 28 shot in a makeshift hospital after Assad's forces entered Jdeidet al-Fadel. We fear that the victims of the massacre are much higher."

The Syrian Observatory for Human Rights, an opposition monitoring group operating from London, said it documented 80 names of people killed, including three children, six women and 18 rebel fighters.

The Syrian Network for Human Rights said the International Committee for the Red Cross should be allowed to evacuate civilians from the district after credible reports of "extrajudicial killings and summary executions inside homes and tens of cases of sexual violence."

Syrian state television showed troops in a pickup truck patrolling the dusty town and several bodies of dead men in front of a building that appears to have been wrecked by gunfire. A Syrian commander described them as "terrorists."

Video footage taken by activists showed three bodies of young men lying next to each other in what appeared to be a makeshift clinic, all with apparent bullet wounds. 

In a pattern seen in other towns and neighborhoods overrun by Assad's forces, activists said shops in Jdeidet al Fadel were looted and torched.

Assad's forces have been accused of massacring hundreds of Sunnis in areas they stormed in Hama and Homs provinces and Damascus suburbs, while international rights groups say rebel forces have also committed atrocities, although on a smaller scale.

ZAATARI REFUGEE CAMP, Jordan - A sniper’s bullet ripped through Hazem Mahmoud’s back seven months ago in Homs, Syria. The 15-year-old has felt nothing below his waist since then. His legs are pale, wasted and scarred by sores.  One bites deep into his thigh.

“Before the war life was sweet,’’ Hazem said as he lay in a tin hut next to his sleeping sister. “Then the bombs and the shooting started. Now there are no hospitals in Syria, no one to help me.’’

Mohammad Hannon / AP, file

Refugees walk through water and mud in Zaatari refugee camp near the Syrian border in Mafraq, Jordan, on Jan. 8.

A wheelchair and a single suitcase are the family’s sole possessions in the camp. The family discarded everything else along the way.

Hazem is a boy the world has all but forgotten. At the Zaatari camp he is only one of thousands of desperate new arrivals on a recent morning. Only when we alert the United Nations staff is an ambulance summoned.

In the overcrowded camp, medical services are overwhelmed, and aid running dangerously short. Humanitarian officials estimate that more than 1.2 million Syrians have fled the country to escape the war between President Bashar Assad’s forces and the largely Sunni rebels trying to unseat him.

All the aid agencies complain they are approaching a funding crisis as big as the camp itself. Just one example: The United Nations Children's Fund (UNICEF) provides the water for 120,000 refugees in Zaatari. Three hundred huge tankers lumber through the gates each day.

“This is for drinking, for washing, for the toilets, and yet we are not in a position to renew the contracts to keep that water coming,’’ says Simon Ingram of UNICEF.

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Many aid workers report tensions are mounting among the refugees; scuffles are common. Violent protests are no longer rare.

It doesn’t help that the huge sums promised by the international community have not been fulfilled. More than two-thirds of the funds needed to cover the basic needs of Syrian refugees have not materialized, United Nations officials say.

The result on the ground? Even food hand-outs might have to be cut, says Laure Chadraoui of the World Food Program.

"There is a lot of anger here. The assistance we provide helps hold that in. Take it away and the pressure cooker will explode,’’ she said.

Invisible wounds

Hazem and his family’s escape from Syria was both exhausting and miraculous.

After that his family – mother, father, sister and sister – moved from safe house to safe house, dodging Syrian army checkpoint.

Finally they were smuggled out of the city; first to Iraq and finally to Jordan.

The final stretch of the journey was 250 miles, with Hazem at times hoisted on his father’s back.

“What could I do, leave him to die?’’ said the father, who kept his face hidden around journalists.

The family’s story is one of thousands. Many times the wounds aren’t visible.

Ibrahim, a serious-faced boy of 13, says he dreams of joining his four brothers who fight with the rebel army.

His nightmares are more real, about the day his home was bombed and he saw his friend shot dead.

“He was just in front, it could easily have been me,’’ Ibrahim said.

There is help. Ibrahim attends a school funded by UNICEF and a therapist helps him deal with his terrifying memories.

Doctors will not be able to help Hazem walk again. The news is not good when he was finally taken to Jordanian medical center in the camp. His spinal cord is severed, a doctor says.

“If we had a chance to treat when this first happened, maybe we could have helped. But it’s too late now,’’ said Dr. Ahmad A’Sanah.

Hazem at least will live, when so many have died. But what kind of life among the refugees of Zaatari is hard to imagine.

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The Syrian regime has carried out indiscriminate and sometimes deliberate airstrikes against civilians that have killed at least 4,300 people since last summer and that amount to war crimes, an international human rights group said Thursday.

Human Rights Watch said Syrian fighter jets have deliberately targeted bakeries, bread lines and hospitals in the country's northern region.

Parts of northern Syria — especially areas along the border with Turkey — have in the past months fallen under the control of rebels fighting to topple President Bashar Assad, including several neighborhoods of the northern city of Aleppo, the country's largest urban center.

"The aim of the airstrikes appears to be to terrorize civilians from the air, particularly in the opposition-controlled areas where they would otherwise be fairly safe from any effects of fighting," Ole Solvang of the New York-based group told The Associated Press.

These attacks are "serious violations of international humanitarian law," and people who commit such breaches are "responsible for war crimes," the group said.

Aleppo Media Center AMC via AP

A citizen's image authenticated by The Associated Press shows homes destroyed in what was said to be a government airstrike and shelling in Aleppo, Syria, on Thursday. A rights group has accused Syria of indiscriminate and even deliberate airstrikes against civilians.

Solvang led the HRW team that inspected 52 sites in northern Syria and documented 59 unlawful attacks by the Syrian Air Force. At least 152 people were killed in these attacks, according to an HRW report released Thursday.

In most of the strikes, the regime planes appear to have had no military target in sight, such as armed opposition supporters or rebel headquarters, when they dropped their weapons on civilian areas, the group said.

The 80-page HRW report said that across Syria, more than 4,300 civilians have been killed in attacks by Assad's jets since last July.

The report is the most comprehensive study of Syrian air force operations since last summer, when Assad's forces started to rely heavily on fighter jets to repel rebel advances and reverse their territorial gains.

Officials in Damascus could not immediately be reached for comment on the report.

The opposition now controls large swaths of northern Syria, and last month captured their first provincial capital, the city of Raqqa. Opposition fighters also control whole districts of Aleppo and some key infrastructure in the east, including oil fields and dams on the Euphrates River.

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While the rebels have made major gains, they often cannot hold on to the territory because of the regime's superior air power. The continued threat from the air has also stalled efforts to effectively govern rebel-held areas, allowing opposition leaders from the Western-backed alliance only brief excursions into areas under rebel control.

For its part, the Syrian National Coalition has been marred by severe divisions in its ranks since its formation late last year in Qatar, and its leaders are mostly seen as disconnected from the myriad rebel forces fighting inside Syria.

U.S. Secretary of State John Kerry met with opposition leaders in London on Wednesday to discuss ways to step up aid to rebels fighting to topple the regime in Damascus.

The United Nations says that more than 70,000 people have been killed in Syria's 2-year-old conflict. It started with peaceful protests against Assad's rule, inspired by other Arab Spring uprisings, but following a harsh regime crackdown descended into full-blown civil war.

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Iraqi al Qaeda and Syria militants announce alliance

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Rebels claim Assad forces fired chemical weapon

Stringer / Reuters

A fighter from the Syrian rebel group Jabhat al-Nusra holds an Islamist flag in Raqqa province, eastern Syria, in this March 12, 2013 photo. The Iraqi wing of al-Qaeda announced on Tuesday that Nusra was now its Syrian branch and the two groups would operate under one name -- the Islamic State in Iraq. The flag reads "There is no God but Allah, and Muhammad is the messenger of Allah."

Al Qaeda's branch in Iraq said it has merged with Syria's extremist Jabhat al-Nusra, a move that shows the rising confidence of radicals within the Syrian rebel movement and is likely to trigger renewed fears among its international backers.

A website linked to Jabhat al-Nusra confirmed on Tuesday the merger with the Islamic State of Iraq, whose leader, Abu Bakr al-Baghdadi, first made the announcement in a 21-minute audio message posted on militant websites late Monday.

Jabhat al-Nusra has taken an ever-bigger role in Syria's conflict over the past year, fighting in key battles and staging several large suicide bombings. The U.S. has designated it a terrorist organization. 

The Syrian group has made little secret of its links across the Iraqi border but until now it has not officially declared itself to be part of al Qaeda.

Al-Baghdadi said that his group — the Islamic State of Iraq — and Syria's Jabhat al-Nusra will now be known as the Islamic State in Iraq and the Levant.

"It is time to announce to the Levantine people and the whole world that Jabhat al-Nusra is merely an extension and part of the Islamic State of Iraq," he said.

He said that the Iraqi group was providing half of its budget to the conflict in Syria. Al-Baghdadi said that the Syrian group would have no separate leader but instead be led by the "people of Syria themselves" — implying that he would be in charge in both countries.

The formal merger of such a high-profile Syrian rebel group with al Qaeda is likely to spark concerns among backers of the opposition who are enemies of the global terror network, including both Western countries and Gulf Arab states.

It may increase resentment of Jabhat al-Nusra among other rebel factions. Rebels have until now respected the radical group's fighters for their prowess on the battlefield, but a merger with al Qaeda will complicate any effort to send arms to rebels from abroad.

Website confirms
A website linked to Jabhat al-Nusra known as al-Muhajir al-Islami — the Islamic emigrant — confirmed the merger.

The authenticity of neither message could be independently confirmed, but statements posted on major militant websites are rarely disputed by extremist groups afterward.

Jabhat al-Nusra emerged as an offshoot of Iraq's al-Qaeda branch in early 2012, as one of a patchwork of disparate rebel groups in Syria.

One of the most dramatic attacks by the group came on March 4, when 48 Syrian soldiers were killed in a well-coordinated ambush after seeking refuge across the frontier in Iraq following clashes with rebels on the Syrian side of the border. The attack occurred in Iraq's restive western province of Anbar, where al-Qaeda is known to be active.

A top Iraqi intelligence official told The Associated Press in Baghdad that Iraq has always known that "al Qaeda in Iraq is directing Jabhat al-Nusra."

He said they announced their unity because of "political, logistical and geographical circumstance." The official said Iraqi authorities will take "strict security measures to strike them."

In an editorial published Tuesday in the Washington Post, Iraqi Prime Minister Nouri al-Maliki warned that a "Syria controlled in whole or part by al Qaeda and its affiliates — an outcome that grows more likely by the day — would be more dangerous to both our countries than anything we've seen up to now."

Iraqi officials say the jihadi groups are sharing three military training compounds, logistics, intelligence and weapons as they grow in strength around the Syria-Iraq border, particularly in a sprawling region called al-Jazeera, which they are trying to turn into a border sanctuary they can both exploit. It could serve as a base of operations to strike on either side of the border.

Baghdad officials said last week they have requested U.S. drone strikes against the fighters in Iraqi territory. A U.S. official confirmed that elements within the Iraqi government had inquired about drone strikes. But the official said the U.S. was waiting to respond until the top level of Iraqi leadership makes a formal request, which has not happened yet.

All officials spoke anonymously as they were not authorized to give official statements to the media.

Eastern Syria and western Iraq have a predominantly Sunni Muslim population like most of the rebels fighting Syrian President Bashar Assad, who belongs to the minority Alawite sect, an offshoot of Shiite Islam. The Baghdad government is dominated by Shiites, who are a majority in Iraq.

Suicide bombing in Damascus
The announcement came hours after a suicide car bomber struck Monday in the financial heart of Syria's capital, Damascus, killing at least 15 people and damaging the nearby Central Bank.

No one has claimed responsibility for the attack, but such operations have been claimed by Jabhat al-Nusra in the past.

State-run Al-Ikhbariyeh TV quoted Central Bank Gov. Adib Mayaleh as saying the bank returned to work as usual at 1 p.m. Tuesday (1000 GMT) "despite the destruction" caused by the bombing.

The Syrian Foreign Ministry sent two letters to the United Nations and the U.N. Security Council protesting the Damascus explosion, blaming "terrorists" who "receive financial and logistic support from regional states and other foreign nations."

The Syrian National Coalition, the country's main opposition group, blamed Assad's regime for the bombing, saying, "The intent is clearly to terrorize the people." It said the area where the explosion occurred is heavily guarded.

Elsewhere on Tuesday, activists reported violence in different parts of Syria.

State-run news agency SANA said one person was killed and two others wounded when mortar shells struck the upscale Damascus district of Kafar Souseh. Two other mortars crashed onto the roofs of residential buildings in the al-Qassaa district, causing material damage but no casualties.

The Britain-based Syrian Observatory for Human Rights reported air raids on suburbs of Damascus as well as the northern province of Raqqa and Idlib.

The observatory said that Ali Matar, a local rebel commander in the eastern province of Deir el-Zour, was shot dead in the eastern city of Mayadeen. It did not say who was behind the attack but added that some of his guards were wounded in the shooting.

Syria's crisis, which began in March 2011 with protests calling for Assad's ouster, then evolved into a civil war. The U.N. says more than 70,000 have been killed in the conflict.

Youssef Badawi / EPA

Burned cars seen at the site of what Syrian authorities said was a suicide car bombing in Damascus on Monday. At least 15 people were reported killed and 53 wounded in the blast. The government blamed 'terrorists,' and Syrian rebels blamed the government.

A suicide car bomb exploded in the main business district of Damascus on Monday, killing at least 15 people, setting cars ablaze and damaging buildings, according to state television.

A Damascus resident who described the blast as the biggest she had heard in the capital during the two-year-old revolt against President Bashar Assad said large plumes of black smoke were rising from the Sabaa Bahrat district.

State television said the explosion had occurred near a school in Sabaa Bahrat, a heavily populated area that also houses the Central Bank and the Finance Ministry. It said 53 people were wounded.

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Residents and opposition activists reported hearing gunfire and ambulance sirens in the vicinity. State television said shots had been fired in the air to clear a path for ambulances.

It showed footage of seven bodies in the street, including at least two charred corpses in the wreckage of an overturned bus. The fire brigade was dousing flames from cars crushed by the blast. Other vehicles were still on fire, lined up in what appeared to be a car park.

Men carried away a woman on a stretcher whose face was covered in blood. Panic-stricken women in long black dresses and headscarves ran toward the scene. State television showed some bandaged children in school uniform.

The British-based Syrian Observatory for Human Rights, a monitoring group with a network of local sources, including hospitals, said at least eight people had been killed.

There was no immediate claim of responsibility for the attack, but state media blamed "terrorists," a term the government uses for opposition fighters. Opposition groups accused the government of carrying out the attack.

Syrian insurgents based in the outskirts of Damascus have pushed into areas near the government-held heart of the city, stepping up mortar and car bomb attacks in recent weeks.

More than 70,000 people have been killed in the conflict, which started with peaceful protests against four decades of Assad family rule that were violently suppressed. An armed struggle ensued, forcing more than a million Syrians to flee abroad, and displacing millions more inside the country.

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Rebels ask US to shoot down Assad's warplanes

REUTERS/Paul J. Richards/Pool

U.S. Secretary of State John Kerry talks to reporters after finding out that the aircraft had a mechanical failure before take off, at the Andrews Air Force Base in Maryland April 6, 2013.

U.S. Secretary of State John Kerry will press Turkey on Sunday to quickly normalize relations with Israel, keep its border with Syria open to refugees and improve ties with Iraq, a senior U.S. official said. 

Kerry arrived in Istanbul some two weeks after U.S. President Barack Obama brokered a rapprochement between Turkey and Israel, whose relations were shattered by the killing of nine Turkish citizens in a 2010 Israeli naval raid on a Gaza-bound flotilla. 

The rapprochement could help regional coordination to contain spillover from the Syrian civil war and ease Israel's diplomatic isolation in the Middle East as it faces challenges posed by Iran's nuclear program. 

Despite Obama's having pulled off a diplomatic coup on March 22 - a three-way telephone call with the Israeli and Turkish prime ministers, who had not spoken since 2011 - Washington has some concerns that Turkey might be backtracking on the deal. 

Israel bowed to a long-standing demand by Ankara, once its close strategic partner, to apologize formally for the deaths aboard the Turkish vessel Mavi Marmara. It was boarded by Israeli marines who had intercepted a flotilla challenging Israel's naval blockade of the Palestinian-run Gaza Strip. 

Israeli Prime Minister Benjamin Netanyahu's office said he had agreed to conclude an agreement on compensation and that he and Turkish Prime Minister Tayyip Erdogan agreed to normalize ties, including returning their ambassadors to their posts. 

A senior U.S. official told reporters traveling with Kerry that he "will encourage Turkey to expeditiously implement its agreement with Israel and fully normalize their relationship to allow for deeper cooperation between the two countries." 

While the official denied the United States was worried the Turkish government might be backing away from the deal, another U.S. official earlier this week said Washington was concerned. 

Kerry will also raise Syria and Iraq during his talks on Sunday with Erdogan and Turkish Foreign Minister Ahmet Davutoglu in Istanbul, his first stop on a 10-day trip to the Middle East, Europe and Asia. 

One of the underlying motivations for the Israeli-Turkish rapprochement, at least on the Israeli side, has been a desire to secure allies in the region as the Syrian civil war churns into its third year. 

Kerry's message in Istanbul will include "reiterating the importance of keeping the borders open to Syrians fleeing from violence," the senior U.S. official told reporters with Kerry. 

The official said this was a reference to reports, which Turkey denied on March 28, that it had rounded up and deported hundreds of Syrian refugees following unrest at a border camp. 

Witnesses said hundreds of Syrians were bussed to the border after clashes in which refugees in the Suleymansah camp, near the Turkish town of Akcakale, threw rocks at military police, who fired teargas and water cannon. 

Turkey's foreign ministry said 130 people, identified as being "involved in the provocations," crossed back into Syria voluntarily, either because they did not want to face judicial proceedings or because of repercussions from other refugees. 

The incident highlighted the strain that the exodus from Syria's civil war is placing on neighboring states. 

Since the revolt in Syria began two years ago, more than 1.2 million Syrians fleeing violence and persecution have registered as refugees or await processing in neighboring countries and North Africa, according to U.N. figures. 

They include 261,635 in Turkey, mostly staying in 17 camps, many of them teeming. 

Kerry also plans to nudge Turkey to improve ties with Iraq, which is troubled by efforts by its autonomous Kurdistan region, where ethnic Kurds have administered their affairs since 1991, to sell energy to Turkey. 

The Iraqi central government argues that this would deprive it of oil revenues that belong to Iraq as a whole.