Syrian rebels fighting to topple President Bashar Assad said Friday that Damascus International Airport was now a "legitimate target” and warned civilians going there did so "at their own risk."

The announcement comes as fighting between Assad's troops and rebels intensified near the airport just south of the capital.

Clashes in the area forced the closure of the airport road for the second time this week.

The fighting has also forced the suspension of commercial flights in the past week.

Two fighters operating in the capital's southern suburbs said the rebels were trying to besiege the airport in an attempt to cut military supplies to the regime. The two spoke to The Associated Press on condition of anonymity for security reasons.

Endgame for Assad?
Western opponents of Assad suggested that an endgame was approaching in the 20-month-old conflict that has killed 40,000 people.

"Events on the ground in Syria are accelerating, and we see that in many different ways," U.S. Secretary of State Hillary Clinton said before talks on Thursday with Foreign Minister Sergei Lavrov of Russia, which has backed Assad.

"The pressure against the regime in and around Damascus seems to be increasing," Clinton said in Dublin.

Related content:

Slideshow: Syria uprising
US officials: Syria loads chemical weapons into bombs; military awaits Assad's order
Syria regime 'reeling, armed to the teeth' with chemical weapons
More weapons in Syria could trigger 'all-out war'
Defense chief: Intel 'raises serious concerns' about Syria chemical weapons
Safe exit for Syria's Assad 'could be arranged,' says British prime minister

Syria's government says that is not the case, and that the army is driving rebels back from positions in the suburbs and outskirts of Damascus where they have tried to concentrate their offensive.

Many who have followed the events on the ground say talk of an endgame is overblown or premature.

"I think it's unreasonable to expect that the battle is in its last stages right now," said Rami Abdelrahman of the Syrian Observatory for Human Rights, which has tracked the fighting since it began in March 2011.

"The big advances are only in the media. The situation is certainly not good, for anyone. The Syrian economy is dead. But conditions for the rebels are not good either. ... Rebel-held parts of Aleppo are barely eating and are always at risk of army shelling,” he added.

"It is true however that the regime is withdrawing from many areas ... and the regime is being exhausted," he told Reuters.

Airport 'under siege'
Cutting access to the airport —just 12 miles from the city center — would be a symbolic blow. The rebels acknowledge it is still in army hands.

"The rebel brigades who have been putting the airport under siege decided yesterday that the airport is a military zone," said Nabil al-Amir, a spokesman for the rebels' Damascus Military Council. "The airport is now full of armored vehicles and soldiers."

"Civilians who approach it now do so at their own risk," he said.

Fighters had "waited two weeks for the airport to be emptied of most civilians and airlines" before declaring it a target, he added.

He did not say what they would do if aircraft tried to land. A rebel spokesman on Thursday said fighters would not "storm the airport but we will blockade it."

Foreign airlines have suspended all flights to Damascus since fighting approached the airport in the past week, although some Syrian Air flights have used the airport in recent days.

The Associated Press and Reuters contributed to this report.

More world stories from NBC News:

• 'It pains me': Clinton decries plight of women in male-dominated countries
• Hamas leader returns to Palestinian territories for first time since 1967
• PhotoBlog: Shark fins from Canada sold as delicacy in China
• EXCLUSIVE: US behind Afghan 'insecurity,' Karzai says
• ANALYSIS: After 10 years of Karzai rule, has life improved in Afghanistan?
• Sex mobs target Egypt's women
• Africa's lion population plummets, study finds
• ANALYSIS: Egyptians warn Morsi is no friend of US

Follow World News from NBCNews.com on Twitter and Facebook

Evert Elzinga / EPA

A site at Schiphol airport where an unexploded World War II bomb was found during excavation works on Wednesday.

Parts of Amsterdam's Schiphol international airport – one of Europe's busiest aviation hubs – were shut down Wednesday after workers found an undetonated bomb during routine construction work.

"This will most likely have an impact on flight routine at our airport and could lead to delays and cancellations," an airport official told NBC News.

Workers found the explosive device during construction work on a new hydrant system to be used for re-fueling aircraft.

Experts blow up 550-pound WWII bomb found in Munich

Schiphol is one of Europe's busiest airports and handles approximately 50 million passengers annually.

A statement on its website said: "The bomb squad is investigating at the moment. This may have implications for air traffic in the form of cancellations and delays."

The Brussels-based main European air traffic control agency, Eurocontrol, posted on Twitter that passengers could expect "major delays."

The find comes only a day after experts in Munich triggered a controlled explosion of a 550-pound American WWII bomb in the center of Munich.

"A bomb disposal team with experts is presently assessing the situation, which will determine how long we will need to keep the section of the terminal closed," Cora Koopstra, from the airport's "action team," told NBC News.

The device was discovered at "Pier C," the wing of the terminal used mainly by flights to and from the European Union's passport-free Schengen zone. The terminal is a busy hub for European travelers and those connecting to Schengen destinations from international flights such as those from the U.S.

During World War II, Nazi Germany used the airport as a base for air raids on Britain. In 1943, the airport was destroyed by allied fighter aircraft; 400 tons of U.S. bombs were dropped on the complex.

More world stories from NBC News:

• 'Superhuman' athletes burst onto world stage
• Red Cross halts most Pakistan aid in wake of beheading
• Unexploded WWII bomb disrupts Amsterdam airport
• Pakistani Christians live in fear after girl's blasphemy arrest
• 'A less polar pole': Arctic sea ice at record low
• Botched restoration turns Spanish church into tourist attraction

Follow World News from NBCNews.com on Twitter and Facebook

Updated at 1:37 p.m. ET: SOFIA, Bulgaria -- A bombing that killed at least seven people and injured dozens on a bus full of Israeli tourists was most likely a suicide attack, Bulgarian officials said Thursday. The suspected attacker was carrying a fake Michigan driver's license, they added.

Interior Minister Tsvetan Tsvetanov said the suspect appeared on security camera tape near the bus for nearly an hour before the attack that gutted the airport in Burgas, a popular gateway for tourists visiting the Black Sea coast

"We have established there was a person who was a suicide bomber in this attack (on Wednesday)," Tsvetanov told reporters. "This person had a fake driving license from the United States, from the state of Michigan. He looked like anyone else -- a normal person with Bermuda shorts and a backpack."

Bulgarian media reported Thursday that former Guantanamo Bay detainee Mehdi Ghezali was believed to be the suicide bomber. However, U.S. intelligence officials later denied the reports.

Video footage showed the suspect wearing checked shorts and a blue T-shirt. He appeared to be Caucasian with long dark curly shoulder-length hair under a dark blue baseball cap. 

The bomber was said to be 36 years old and had been in the country for between four and seven days before the attack, Reuters reported.

Officials are still trying to determine how the alleged bomber triggered the explosion. 

"He either had turned with his backpack toward the bus when he exploded it or pretended he was one of the group putting his backpack in the baggage compartment under the bus," according to a Bulgarian official with knowledge of the investigation who spoke with the New York Times.  "Video footage clearly shows him in the airport earlier wandering back and forth, following the group, looking nervous."

Authorities had managed to obtain DNA samples from the fingers of the suspected bomber, Tsvetanov said.

Officials did not release the name that appeared on the fake driver's license. 

Prime Minister Boiko Borisov added: "We worked on this with colleagues from the FBI and CIA. They said that there is no such person in their database."

According to the Associated Press, officials lowered the death toll to seven, including the suspected bomber, after mistakenly reporting that someone had died overnight.

Bulgarian security services had received no indications of a pending attack. However, Israel accused Iranian-backed Hezbollah militants of responsibility.

Iran denied it was behind Wednesday's bombing.

Mangled metal
The tourists had just arrived in Bulgaria on a charter flight from Israel and were on the bus in the airport parking lot when the blast tore through the double-decker. Body parts were strewn across the ground, mangled metal hung from the vehicle's ripped roof and black smoke billowed over the airport.

"It felt like an earthquake and then I saw flying pieces of meat," said Georgi Stoev, an airport official. "It was horrible, just like in a horror movie."

On Thursday, the airport in Burgas -- a city of some 200,000 people at the center of a string of seaside resorts -- remained closed and police prevented people from approaching.

Israeli Defense Minister Ehud Barak accused the Tehran-backed Lebanese Shiite Muslim group Hezbollah of carrying out the bombing. "The immediate executors are Hezbollah people, who of course have constant Iranian sponsorship," Barak told Israel Radio.

Israeli Prime Minister Benjamin Netanyahu also said Iran, the Jewish state's arch-enemy, was behind the attack and that "Israel will react powerfully against Iranian terror."

Israeli government spokesman Mark Regev linked the arrest of a foreigner in Cyprus earlier this month on suspicion of plotting an attack on Israeli tourists there with the Bulgaria bombing.

"The suspect who was arrested in Cyprus, in his interrogation, revealed an operational plan that is almost identical to what happened in Bulgaria. He is from Hezbollah ... this is a further indication of Hezbollah and Iran's direct responsibility," he told Reuters.

Bangkok blasts wound Iranian attacker, 4 others

The blast occurred on the 18th anniversary of a bomb attack at the headquarters of Argentina's main Jewish organisation that killed 85 people and the Argentine government blamed on Iran, which denied responsibility.

BGNES via AFP - Getty Images

Smoke rises over Burgas airport following a Wednesday's blast.

Israeli officials had previously said that Bulgaria, a popular holiday destination for Israeli tourists, was vulnerable to attack by Islamist militants who could infiltrate via Turkey.

Israeli diplomats have been targeted in several countries in recent months by bombers who Israel said struck on behalf of Iran.

'Inexcusable'
Although Tehran has denied involvement, some analysts believe it is trying to avenge the assassinations of several scientists from its nuclear program that the Iranians have blamed on Israel and its Western allies.

Israel and Western powers fear Iran is working towards a nuclear bomb but it says its uranium enrichment work is strictly for peaceful ends. Both Israel and the United States have not ruled out military action against Iranian nuclear facilities.

"The attack is terrible and inexcusable," said German Foreign Minister Guido Westerwelle. "It is a time to act responsibly. We have no information of our own. We urge caution in starting to assign blame."

The Associated Press and Reuters contributed to this report.

More world stories from NBC News:

• Experts say 'non' to Champagne as English wine sparkles
• Bombing kills Syrian ministers at heart of Assad rule
• North Korean leader 'awarded' top military rank
• US official: Up to $8 billion wasted rebuilding Iraq
• NBC's Ayman Mohyeldin answers your questions on Syria
• Video: Security fiasco flares ahead of Olympics

Follow World News on NBCNews.com on Twitter and Facebook

Updated at 6:04 p.m. ET: SOFIA, Bulgaria -- An explosion on a bus carrying Israeli tourists at an airport in Burgas killed at least six people and injured 32 others, Bulgarian authorities said. Bulgarian officials could not confirm the deadly blast was terror-related but Israeli Prime Minister Benjamin Netanyahu blamed Iran.

"Iran is responsible for the terror attack in Bulgaria, we will have a strong response against Iranian terror," said Netanyahu in a statement, according to Haaretz, an Israeli newspaper.

Tehran did not immediately issue a comment.

A bomb caused the explosion, Bulgarian Foreign Minister Nikolai Mladenov told Reuters.

The Israelis had landed at the airport around 4:45 p.m. local time (9:45 a.m. ET) and boarded a bus to their hotel when the explosion happened, The Jerusalem Post reported. Body parts were flung onto the tarmac and a thick black plume of smoke rose above the airport.

An 11-year-old Israeli girl and two pregnant women were among those injured, according to Focus, a Bulgarian news agency.

EPA

Smoke rises over the Burgas airport in Bulgaria, after an explosion on Wednesday.

"I do not know what it was, but it was a very powerful blast, and I think it was something placed on purpose in the bus, which carried 47 Israeli tourists," Burgas mayor Dimitar Nikolov told BTV television. Burgas is 250 miles from Sofia, Bulgaria's capital.

Nikolov said 171 people had arrived on a plane from Israel to spend their holiday at the Black Sea coast. One American and one Slovenian passenger were on board, he said.

The Bulgarian Press Office, which provided the casualty figures, said only one bus was involved in the explosion, but added the investigation is ongoing. 

According to a Bulgarian news service, an eyewitness named Daniel told the Voice of Israel radio program: “I was literally watching people crawling out of the bus. They were screaming and one of them had no arms or legs. It was horrible.”

Another Israeli traveler told the radio station: “The people who survived got through the windows and were trying to crawl over the bodies. The bus was destroyed from both sides.”

In separate statements, President Barack Obama, Secretary of State Hillary Clinton and United Nations Secretary-General Ban Ki-moon condemned the attack.

Reuters

An explosion damaged a bus carrying Israeli tourists at Burgas Airport in Bulgaria on Wednesday.

"As Israel has tragically once more been a target of terrorism, the United States reaffirms our unshakeable commitment to Israel's security, and our deep friendship and solidarity with the Israeli people," Obama said. He called Netanyahu on Wednesday to express his condolences.

Clinton said the U.S. is prepared to offer "any assistance necessary" and that she was prepared to "work with our partners in Bulgaria, Israel and elsewhere so that the perpetrators can be apprehended swiftly and brought to justice for this appalling crime."

Wednesday's bombing coincided with the 18th anniversary of the bombing of a Jewish community center in Argentina that killed 85 people. According to the BBC, Argentinian prosecutors charged Iran with orchestrating the attack, which they believe was carried out by Hezbollah, the Lebanon-based militia. 

Netanyahu said Israel would respond.

"All the signs lead to Iran. Only in the past few months we have seen Iranian attempts to attack Israelis in Thailand, India, Georgia, Kenya, Cyprus and other places," Netanyahu said in a statement. "This is an Iranian terror attack that is spreading throughout the entire world. Israel will react powerfully against Iranian terror."

But Jerusalem Post writer Yaakov Katz questioned the connection between the two events.  

"While the attack is severe, it is not of the scale of what happened in 1994," Katz wrote. In 1994 in Argentina, a van filled with explosives rammed into the Jewish community center, killing 84 people. Wednesday's attack, he said, appeared to have been perpetrated by a suicide bomber or a planted bomb. 

"This is a break from Hezbollah's traditional tactic of carrying out attacks with less of a footprint," Katz wrote. "In previous plots that were thwarted recently, there were attempts to shoot down Israeli airliners with shoulder-to-air missiles, to plant bombs on diplomatic cars or to assassinate Israeli diplomats. Nothing that would leave evidence behind."

Israeli officials had previously said that Bulgaria, a popular holiday destination for young Israeli tourists, was vulnerable to attack by Islamist militants who could infiltrate via nearby Turkey.

Israeli diplomats have been targeted in several countries in recent months by bombers who Israel said struck on behalf of Iran.

Though Tehran has denied involvement, some analysts believe it is trying to avenge the assassinations of several scientists from its controversial nuclear program, which the Iranians have blamed on Israel and its Western allies.

Israel has threatened air strikes against Iran's nuclear facilities if diplomatic efforts fail to stop Tehran getting nuclear weapons, which it denies it is seeking.

The Israel Airports Authority announced disruptions in flights from Israel and Europe, according to Haaretz.

NBC's Lawahez Jabari in Jerusalem, Reuters and The Associated Press contributed to this report.

More world stories from NBC News:

• Bombing kills Syrian ministers at heart of Assad rule
• US official: Up to $8 billion wasted rebuilding Iraq
• 'Mystery woman' stirs talk of change in North Korea
• Video: Security fiasco flares ahead of Olympics

Follow World News on NBCNews.com on Twitter and Facebook

MEXICO CITY -- Three policemen died in a shootout with two other officers suspected of drug trafficking at Mexico City's airport on Monday, as panicked travelers scrambled for cover in the busy facility.

The shootout occurred when three federal officers approached the two suspects in the airport's Terminal 2, which handles international and domestic flights. Two agents were killed at the terminal and another later died of his injuries in hospital.

More than a dozen shots were heard, Milenio Television reported. Television footage showed a body lying on the floor of the terminal in what appeared to be a publicly accessible area of the airport.

Three shots rang out at first, witness Israel Lopez, a 23-year-old Mexico City student who had gone to the airport to see off a friend, told The Associated Press. Lopez didn't see who those shots were directed at, but then the gunfire came closer.

Alfredo Estrella / AFP - Getty Images

Federal Police officers stand guard at an entrance near the fast-food area of Benito Juarez international airport Terminal 2, in Mexico City, where two police officers were shot dead and a third was wounded on Monday. Airport spokesman Jorge Andres Gomez said authorities are going through the security cameras to know the exact events of the shooting.

"We were in the food court, and some policemen came in and started shooting at another policeman who was on the floor," Lopez told the AP. "We dove to the floor and covered ourselves with chairs."

Lopez said the shooters wore blue uniforms like those of federal police who provide security at the airport. He said the shooters then ran to the parking area "as if they were pursuing somebody," and he lost sight of them.

'One of the safest places'
Robert Gray, an evangelical missionary from Hart, Michigan, but who has lived with his family in the city of Puebla for the past four years, arrived at the airport after the shooting with his wife, two daughters and son to catch a flight back to the U.S. to visit family.

"It's surprising to see it happening at the airport. It's one of the safest places in the city," Gray told the AP.

The suspects, who remain at large, are believed to be part of a larger group of officials involved in a cocaine ring, Mexico's security ministry said in a statement.

"The Federal Police has identified the two officers who opened fire and several investigative units are now focused on capturing them," the federal Public Safety Department said in a statement.

14 mutilated corpses, threat message to drug cartel found in Mexican city

Airport spokesman Jorge Gomez told Milenio that aircraft departures and arrivals continued normally after the incident.

The airport said in a press statement that the terminal and flights were operating normally following what it described as "a dispute in an open-access area." But the food court remained blocked to public access for hours after the shooting.

Slideshow: Narco culture permeates Mexico, leaks across border

Mexico's drug war is also part of a drug culture with roots in music, movies and even religion

Launch slideshow

Mexico City has seen relatively low murder rates compared to the rest of the country, where drug violence has killed around 55,000 people in the past five-and-a-half years.

Severed heads
But attacks have been creeping up in the capital and its surrounding neighborhoods, with more than 300 gangland killings recorded last year.

Mexico's airports and ports are busy areas for drug smugglers. So far this year federal police have seized more than 440 pounds of cocaine at the capital's airport, double the amount taken there last year.

Alfredo Estrella / AFP - Getty Images

A passenger speaks on her cellphone at Benito Juarez international airport Terminal 2, in Mexico City, where two police officers were shot dead and a third was wounded on Monday.

In 2007, the severed heads of three employees of a customs brokerage firm were found near the airport and in the nearby state of Mexico.

The decapitations were apparently retaliation for the seizure of a half-ton of Colombian cocaine at the airport, officials said at the time.

Mexico got wrong man in high-profile drug arrest 

In 2008, federal police chief Edgar Millan was gunned down inside his Mexico City home, and one of the suspects in that killing had worked as an anti-drug officer at the Mexico City airport.

The suspect had a notebook with detailed information on drug trafficking at the airport, and officials said federal investigations into those operations may have been a key motive for Millan's killing.

Reuters and The Associated Press contributed to this report.

More world news from msnbc.com and NBC News:

• Analysis: Egypt's big turn under the Muslim Brotherhood
• Iraq orders Voice of America, 43 other media outlets to close
• Report: Syrian general, dozens of other soldiers defect to Turkey
• Suu Kyi's journey: Heartbreaking tale of personal sacrifice, loss
• Lonesome George, last-of-its-kind Galapagos tortoise, dies
• Naked valkyries? Nudes open German opera season
• UK's queen to hold historic meeting with ex-IRA commander
• PhotoBlog: Glimpses of the escalating conflict in Syria
• 1.5 million children in imminent danger of starvation in W. Africa

Follow us on Twitter: @msnbc_world