BEIRUT (REUTERS) -UN chemical weapons inspectors in Syria met and took samples from victims of an apparent poison gas attack in a rebel-held suburb of Damascus on Monday, after the UN team themselves survived a gun attack on their convoy.

A Syrian doctor told Reuters from the town of Mouadamiya that investigators from the United Nations had crossed the frontline from the centre of the capital, which remains under the control of President Bashar al-Assad's forces.

The doctor said the inspectors took samples from the victims and there was a plan for them to take samples from corpses too.

With Western powers considering military strikes, despite vocal opposition from Mr Assad and his Russian and Iranian allies, any evidence to support rebel claims that government forces fired gas-laden rockets five days ago that killed hundreds of civilians will be a key element in arguments for peace or war.

With speculation mounting that NATO powers might fire cruise missiles to satisfy calls for action to protect Syrian civilians, US Defence Secretary Chuck Hagel said any operation would be coordinated with allies. British Prime Minister David Cameron cut short a holiday to chair a top level security meeting.

"The United States is looking at all options regarding the situation in Syria. We're working with our allies and the international community," Mr Hagel told a news conference in Jakarta.

"We are analysing the intelligence. And we will get the facts. And if there is any action taken, it will be in concert with the international community and within the

framework of legal justification."

Mr Hagel plans discussions with his British and French counterparts, a senior US official said.

The French foreign minister said on Monday that Russian and Chinese vetoes in the UN Security Council may make it hard to get a UN agreement to satisfy international law. His British counterpart, however, said that should not prevent a response to the worst poison gas attack in 25 years.

Menatime, the UN said in a statement that gunmen shot at the first vehicle in the UN team's six-car convoy, damaging it to the point that the team had to stop to find a replacement car.

"The first vehicle of the Chemical Weapons Investigation Team was deliberately shot at multiple times by unidentified snipers in the buffer zone area," it said. "It has to be stressed again that all sides need to extend their cooperation so that the team can safely carry out their important work."

Syrian state television blamed rebel "terrorists" for the shooting, while the opposition blamed it on pro-Assad militiamen.

Syria agreed on Sunday to let the UN inspectors visit the suspect sites.

The United States and its allies say the offer came too late as evidence has probably been destroyed by heavy government shelling of the area since last Wednesday.

Mr Assad, who has been fighting a two-and-a-half-year revolt, said accusations that his forces used chemical weapons were politically motivated and warned the United States against intervening in his country.

"Would any state use chemicals or any other weapons of mass destruction in a place where its own forces are concentrated? That would go against elementary logic. So accusations of this kind are entirely political," he told the Russian newspaper Izvestia in an interview.

"Failure awaits the United States as in all previous wars it has unleashed, starting with Vietnam and up to the present day."

The United Nations said Damascus agreed to a ceasefire while the UN experts are at the site for inspections.

Syria's conflict has so far been met with international deadlock. The growing violence has killed more than 100,000 people, stoked regional sectarian violence, and revived Cold War-era divisions between Western powers and Russia and China Washington has faced growing calls for action in response to Wednesday's attack, which came a year after President Barack Obama declared use of chemical weapons to be a "red line" which would require a firm response.

Russia, Mr Assad's main arms supplier, says rebels may have been behind the chemical attack and said it would be a "tragic mistake" to jump to conclusions over who was responsible.

Its Foreign Ministry said on Monday that it was concerned about a potential US military response and urged Washington to refrain from falling for "provocations".