The White House said Thursday that the U.S. believes "with some degree of varying confidence" the Syrian government has used chemical weapons — specifically the nerve agent sarin — against its own people.

A letter from the White House to members of Congress said the assessment was based on "physiological samples" but called for a United Nations probe to corroborate it and nail down when and how they were used.

"We are continuing to do further work to establish a definitive judgement as to whether or not the red line has been crossed and to inform our decision-making about what we'll do next," a White House official said. 

"All options are on the table in terms of our response," the official added.

Secretary of State John Kerry told reporters at the Capitol that the U.S. believes chemical weapons were used twice, but the letter doesn't specify that.

"Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin," the letter said.

"We do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime," it added.

"Thus far, we believe that the Assad regime maintains custody of these weapons, and has demonstrated a willingness to escalate its horrific use of violence against the Syrian people."

Defense Secretary Chuck Hagel said he had not seen the evidence supporting the assessment, but added that use of chemical agents "violates every convention of war."

Sarin is a man-made nerve agent that has been used in terrorist attacks in Japan and possibly during the Iran-Iraq War in the 1980s, according to the U.S. Centers for Disease Control. In large doses, it can cause convulsions, paralysis and death.

The U.S. has long believed that Syria was stockpiling chemical weapons. Intelligence reports indicate that it has sarin and the nerve agent tabun along with traditional chemicals like mustard gas and hydrogen cyanide. A 2011 CIA report said Syria was also developing the potent nerve agent VX, which could render a city uninhabitable for days.

Syria's information minister, Omran al-Zoubi, said in an interview with Russian TV that the government has not and will not use chemical weapons and blamed potential evidence of their existence on "armed terrorist groups," the state news agency reported.

A spokesman for the rebel Free Syrian Army, Fahd Almasri, claimed Syria has launched chemical attacks in nine places and was poised to do so again at the Lebanon border and in Damascus "when Assad knows he is finished."

"Now is the moment to find a solution very quickly," Almasri told NBC News in a phone interview.

President Obama has said the verified use of chemical weapons by the regime would be a "red line" and a "game-changer" for U.S. and international military intervention in the Syrian civil war.

"Precisely because the President takes this issue so seriously, we have an obligation to fully investigate any and all evidence of chemical weapons use within Syria," said the letter, which was signed by Obama's legislative director, Miguel Rodriguez.

The letter was a response to a request from a bipartisan group of senators who asked the White House for answers after the Israeli military’s top intelligence analyst cited photographs of people "foaming from the mouth” as evidence of chemical weapons use.

Sen. Bob Corker, R-Tenn., ranking member of the Senate Foreign Relations Committee, called the development “deeply troubling.”

“While more work needs to be done to fully verify this assessment…it is becoming increasingly clear that we must step up our efforts,” Corker said.

“I should make clear, however, that it if it comes to the use of military force, before the president takes any action to commit U.S. forces to any effort in Syria or elsewhere, I expect him to fully consult with the Senate and seek an authorization for the use of military force."

Sen. Dianne Feinstein, chair of the Senate Intelligence Committee, said the assessment could spark a dangerous reaction from Damascus.

"I am very concerned that with this public acknowledgement, President Assad may calculate he has nothing more to lose and the likelihood he will further escalate this conflict therefore increases," Feinstein said.

The White House official called for a high level of scrutiny — but also caution.

"Given our own history with intelligence assessments, including intelligence assessments related to weapons of mass destruction, it's very important that we are able to establish this with certainty and that we are able to present information in a way that is airtight," the official said.

NBC News' Kasie Hunt, Kelly O'Donnell, Robert Windrem and Charlene Gubash contributed to this story

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This story was originally published on Thu Apr 25, 2013 11:56 AM EDT

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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Obama warns Syria's Assad not to use chemical weapons

TEL AVIV – American and Israeli defense officials welcomed a new arms sale agreement on Monday as a major step toward increasing Israel’s military strength, but Israeli officials said it still left them without the weapons they would need if they decided to attack Iran’s deepest and best-protected nuclear sites.

Jim Watson / AFP-Getty Images

US Secretary of Defense Chuck Hagel and Israeli Minister of Defense Moshe Yaalon speak during a helicopter tour above the Golan Heights on April 22, 2013. Hagel met his counterpart to put the finishing touches on a major arms deal and for talks on Syria's civil war and the Iranian nuclear threat.

The mixed message came as Defense Secretary Chuck Hagel and his Israeli counterpart, Moshe Yaalon, reaffirmed their commitment to stopping Iran from obtaining nuclear weapons, while sidestepping a continuing disagreement between the two countries about how close to allow Iran to get toward such a goal.

In public, Mr. Hagel again said that Israel had the right to decide by itself how to defend the country, and both officials said military action should be a last resort. But a close adviser to Prime Minister Benjamin Netanyahu said on Monday that “the fundamental difference of views on how much risk we can take with Iran is re-emerging.”

The new weapons sale package includes aircraft for midair refueling and missiles that can cripple an adversary’s air defense system. Both would be critical for Israel if it were to decide on a unilateral attack on Iran.

But what the Israelis wanted most was a weapons system that is missing from the package: a giant bunker-busting bomb designed to penetrate earth and reinforced concrete to destroy deeply buried sites. According to both American and Israeli analysts, it is the only weapon that would have a chance of destroying the Iranian nuclear fuel enrichment center at Fordow, which is buried more than 200 feet under a mountain outside the holy city of Qum.

The weapon, called a Massive Ordnance Penetrator, weighs about 30,000 pounds — so much that Israel does not have any aircraft capable of carrying it. To do so, they would need a B-2 bomber, the stealth aircraft that the United States flew nonstop recently from Missouri to the Korean Peninsula to underscore to North Korea that it could reach their nuclear sites.

The Obama administration has been reluctant to even discuss selling such capability to the Israelis.

Iran has consistently denied that it wants nuclear weapons and has called its uranium enrichment activities peaceful.

The Fordow site has become an increasing source of concern to the Israelis. When they referred last year to Iran entering a “zone of immunity,” Israeli officials said the phrase referred to the moment when the facility would be complete, and immune from attack by Israeli forces. All the centrifuges that enrich uranium at the site have since been installed, but only about a quarter of them are now operating.

Israel has asked the United States for weapons like the Massive Ordnance Penetrator in the past and has been turned down. American officials declined to say whether the yearlong negotiations with Israel that resulted in the new arms package had included a discussion of the new bomb.

Instead, they pointed to a decision by President Obama to send advanced refueling tanker planes to Israel that would make it possible for the country’s fighter aircraft to reach as far as Iran. A similar refueling capability was turned down during the administration of former President George W. Bush.

The debate is about more than just equipment. Israel’s position has been that Iran cannot be allowed to build up too large a stockpile of medium-enriched uranium that could allow it to then race for a bomb. When Mr. Netanyahu addressed the United Nations in New York last September, he drew a red line across a cartoon picture of a bomb, which aides later said indicated that Iran would not be allowed to amass enough medium-enriched uranium to get enough fuel to make a single weapon.

But most of Iran’s production of that uranium is occurring inside the mountain at Fordow. So far, Iran has stayed just below Mr. Netanyahu’s red line, converting some of the fuel to a metallic form that can be used in a nuclear reactor – but that would take a bit more time to convert back to bomb fuel. To the United States, this has offered up more time for a diplomatic solution. To many Israeli officials, it is a ploy, designed to buy time as Iran installs a new generation of centrifuges that could speed its production.

“It’s all about timetables,” said Dore Gold, the president of the Jerusalem Center for Public Affairs and a member of Mr. Netanyahu’s inner circle of strategists. “If you say the goal is to halt Iran in the enrichment phase, you don’t have much time. If you are waiting for Iran to weaponize” — the position the Obama administration has taken – “maybe you can give it another year or more.”

Mr. Yaalon suggested that there was still time. “There are other tools to be used and to be exhausted, whether it is diplomacy, economic sanctions,” Mr. Yaalon said.

He avoided mentioning another element of the strategy: sabotage of the Iranian program, which has included cyberattacks on enrichment facilities and the assassination of Iranian scientists. He urged support for Iranians who oppose the current government in Tehran, especially in advance of a presidential election scheduled for June.

But without “a credible military option,” Mr. Yaalon warned, “there is no chance” that the Iranian government would curtail its nuclear ambitions.

During a news conference with Mr. Yaalon at the Israeli Ministry of Defense, Mr. Hagel pledged that the United States would sustain its commitment to assuring Israel’s “qualitative military edge,” and he was emphatic in discussing Iran.

“Iran will not be allowed to develop a nuclear weapon,” Mr. Hagel said. “Period.”

That was far more definitive than anything he said in his confirmation hearing. There he talked about a strategy of containing Iran – a strategy that seemed at odds with Mr. Obama’s stated position — before correcting himself for the record to align with the administration’s position.

The United States has promised Israel $3.1 billion in military financial assistance in this fiscal year, the highest amount ever. Mr. Hagel cited the $460 million the United States has already given to Israel for its missile-defense systems and noted the $220 million request for the next fiscal year.

After his meetings in Tel Aviv, Mr. Hagel toured northern Israel by helicopter, crossing into the Golan Heights occupied by Israeli forces. The flight took him within a couple miles of the Syrian side of their disputed border and about 30 miles from the Syrian capital, Damascus.

On Monday evening, Amos Yadlin, the former head of military intelligence in Israel, told the annual conference of Israel’s Institute for National Security Studies, that while any Israeli attack would only delay Iran’s program, “this delay could be important because we may have a regime change.”

Mr. Yadlin, now the executive director of the institute, described the tactical differences between the United States and Israel on dealing with Iran as a “time gap.”

“Israel has defined what the trigger is, what the red line is,” he said. Iran, he concluded, “is already there.”

This story, "No Bunker-Buster Bomb in Israel’s U.S. Arms Deal," first appeared in The New York Times.

Kazuhiro Nogi / AFP - Getty Images, file

Protesters demonstrate against the deployment of Osprey aircraft at Marine Corps Air Station Futenma in Okinawa during a Tokyo rally in November. More protests are planned over the large U.S. military presence on the island prefecture.

TOKYO -- As Japan prepares to celebrate the 61st anniversary of the nation's return to sovereignty and the end of U.S. occupation after World War II, some members of one community are getting ready to protest.

The Pentagon hopes to expand a facility in the seaside village of Henoko, Okinawa, as part of a plan to replace an existing base, and many residents aren't happy about it.

"We would like the United States to take back with them as many of these bases as they can," said Ikuo Nishikawa, an activist and native of Henoko who owns a hardware store.

Kyodo via Reuters, file

A Marine Corps Osprey aircraft flies to land at Futenma air base in crowded Ginowan, Okinawa. Some city residents are bothered by the base, but some residents of the town of Henoko, where an expansion is planned to replace it, are angry as well.

The Pentagon says 38,000 U.S. forces live in Japan, most of them in Okinawa, making up the largest American presence in the increasingly tense Pacific Rim. In addition to the 38,000 on shore, there are 11,000 service members based on ships, 5,000 civilian Defense Department workers and 43,000 family members.

Although Defense Secretary Chuck Hagel earlier this month announced a plan to eventually return more than 2,500 acres of land to Okinawans, the last thing some islanders want to see is a larger base -- even though it would replace an existing one that is near the heart of a bigger city and thus considered by many to be a hazard.

Nishikawa, 69, said he was initially open to the idea of a new base in the village. It might have brought him more business.

But now he is worried, particularly since he started hearing people complain about noise from jets, crimes committed by servicemen and neighborhoods declining as more and more bars opened.

"I thought of it as other people's business," Nishikawa said. "It didn't occur to me how a base could destroy your living environment, how much pain it could cause.

"If you come here, this very area where we swim and catch our fish and shellfish, where we take our children to play, will be transformed into a military base. Even today, the two sides of our community are bases -- on the northern side and on the mountainside. And then with this new base, even our ocean will be occupied by a military base."

Despite the objections, Nishikawa concedes that many people in Okinawa rely on U.S. personnel and their families for their livelihoods and wouldn't think of protesting expansion of a base.

On a larger scale, the United States and Japan see a major presence in the country as critical to the security of both, and they work closely together to maintain it. The April 5 announcement included a promise from Hagel that "the United States will consolidate our forces over time and reduce our impact on the most populated parts of Okinawa."

Nonetheless, the fact that Prime Minister Shinzo Abe is in favor of the Henoko expansion makes him and his government the target for much of the anger vented by Okinawans, some of whom say Abe is simply ignoring them. 

"As someone born and raised here, it's hard to accept," Nishikawa said. "The fact that the Japanese government has pushed through this proposal, it's a mockery against the people of Okinawa."

Okinawa's governor, Hirokazu Nakaima, has no qualms about stating his opinion on the matter. "The people of Okinawa prefecture are greatly dissatisfied," he said during an October panel discussion in Washington. "People have been requesting to relocate the bases for 15 or 16 years … but it's not happening."

Jiji Press / AFP - Getty Images, file

Okinawa Gov. Hirokazu Nakaima, shown speaking to reporters in October, has been vocal in his opposition of U.S. and Japanese government plans to expand a base in the seaside village of Henoko.

However, barring a sudden change of heart by the U.S., Okinawa's leaders or the central government, a fight for the future of Henoko seems certain to rage on, and U.S. forces will continue to be stationed on the island in large numbers in case real battles replace political ones.

There's not much the armed forces can do about the sensitive issue except try to foster good will on Okinawa, said Capt. Richard Ulsh, a Marine Corps spokesman at the Pentagon.

"We do our best to reach out to the people of Okinawa and try to help them understand, one, how important that island itself is to the Asia-Pacific region and, two, how important their support is to us ... [and] the major partner that Japan really is," he said.

All the outreach in the world may not be enough to appease islanders who are angry about bases and angry at their own government.

"As someone from Okinawa, I want to remind [Tokyo] about the last big war," said Nishikawa, the hardware store owner. "In the name of national interest, in order to prevent a battle on the mainland, 200,000 Okinawans were sacrificed.

"With that in mind, why is the government continuing to hurt us still?"

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South Korea's defense ministry on Monday backed away from an official's Sunday statement that North Korea may launch a missile by Wednesday, at which time the North had said it could not guarantee the safety of diplomats in the capital of Pyongyang.

The official’s warning came three days after South Korea’s government said that the North had moved at least one medium-range Musudan missile with “considerable range” to the nation’s eastern border, possibly to perform a test launch. The missile has an estimated range of up to 2,490 miles, which would make it capable of striking American bases in Guam.

“We’re thoroughly preparing for this, leaving all possibilities open,” said Kim Jang-Soo, South Korea’s national security chief, adding that the North's likely goal is to wrench concessions from Seoul and Washington.

But on Monday, South Korea's defense ministry said the movement of vehicles and personnel near North Korea's nuclear test site -- picked up on satellite images -- appeared to be normal activity, refuting speculation that the latest actions point to an imminent atomic test, Yonhap news agency reported.

Escalating tensions between the nuclear-armed North and U.S.-aligned South also forced South Korea’s Joint Chiefs of Staff to announce Sunday that the body’s chairman had delayed a visit to Washington, according to The Associated Press.

Meanwhile, the U.S. military said that a top commander in South Korea had also put off a trip to Washington and that the Pentagon had postponed an intercontinental ballistic missile test slated for next week.

The test was "long planned and was never associated with North Korea to begin with," a senior defense official official said, but added that "given recent tensions on the Korean Peninsula, it's prudent and wise to take steps that avoid any misperception or chance of manipulation, so the test has been postponed."

The test was planned for next week at Vandenberg Air Force Base in California. It would have tested the Minuteman 3 ICBM missile, which has a range of about 8,000 miles, although the exact number is classified.

The weekend developments followed the North Korean military’s ominous warning last Thursday that it had been authorized to attack the U.S. using “smaller, lighter and diversified” nuclear weapons — the latest in a series of threats of war against the U.S. since the United Nations imposed tough sanctions in response to the North's third nuclear test in February.

“The moment of explosion is approaching fast,” the North Korean military said in a statement from an unidentified spokesperson.

“No one can say a war will break out in Korea or not and whether it will break out today or tomorrow,” the Thursday statement said.

U.S. Defense Secretary Chuck Hagel said last Wednesday that North Korea’s provocations represent “a real and clear danger and threat” to domestic security and U.S. interests.

“We are doing everything we can ... to defuse that situation on the peninsula,” Hagel said after delivering a speech at the National Defense University at Fort McNair in Washington, D.C.

North Korea has encouraged foreign ambassadors in the capital of Pyongyang to evacuate the country in order to avoid potential hostilities, according to various diplomatic officials.

But South Korea's Kim Jang-Soo suggested to reporters that the North’s warning to diplomats is likely just an attempt to heighten security fears and extract concessions from South Korea and the U.S.

Top embassies, likewise, have appeared to see the North's message as mere rhetoric, according to The Associated Press.

The roughly two dozen countries with embassies in North Korea had not yet announced whether they would evacuate their staffs, the AP reported.

Washington and Seoul want Pyongyang to resume the six-party nuclear talks that it halted in 2009. China, Russia, and Japan were the other key players in the aborted talks.

NBC News' Courtney Kube and Becky Bratu contributed to this report.

This story was originally published on Sun Apr 7, 2013 12:31 PM EDT