Cameron has been the most vocal critic of Sri Lanka's record on rights during a biennale summit of Commonwealth nations being held in the capital Colombo.

London/Beirut - Britain will not join any military action against Syria after a stunning parliamentary defeat on Thursday of a government motion on the issue, dealing a setback to US-led efforts to punish Damascus over the use of chemical weapons against civilians.

Following a 285-272 vote against a motion by British Prime Minister David Cameron to authorise a military response in principle, British Defence Secretary Philip Hammond confirmed Britain would not be involved in any action against Syria.

Hammond said key ally Washington would be disappointed that Britain “will not be involved”, although adding, “I don't expect that the lack of British participation will stop any action”.

But he told BBC TV that “it's certainly going to place some strain on the special relationship”, referring to ties with Washington.

US officials suggested President Barack Obama would be willing to proceed with limited actions against Syria even without specific promises of allied support because US national security interests are at stake.

“President Obama's decision-making will be guided by what is in the best interests of the United States,” White House spokeswoman Caitlin Hayden said in a statement after the British vote. “He believes that there are core interests at stake for the United States and that countries who violate international norms regarding chemical weapons need to be held accountable.”

She added: “The US will continue to consult with the UK government - one of our closest allies and friends.”

In a briefing with senior lawmakers on Thursday, Obama administration officials said they had “no doubt” chemical weapons were used in Syria and that Syrian President Bashar al-Assad's government had used them, US representative Eliot Engel, who participated in the call, told Reuters.

Britain's Cameron, saying he would not override the will of parliament, said it was clear that lawmakers did not want to see a military strike on the Syrian government to punish it for an alleged chemical weapons attack in the suburbs of Damascus last week.

Asked by Labour leader Ed Miliband whether he would promise not to circumvent parliament and authorise military action, Cameron said: “I can give that assurance. I strongly believe in the need for a tough response to the use of chemical weapons, but I also believe in respecting the will of this House of Commons.”

The parliamentary vote reflected deep misgivings stemming from Britain's involvement in the 2003 invasion of Iraq.

In comments before the parliamentary vote, a White House spokesman said the United States had been encouraged by the support voiced by a wide variety of world leaders, such as the Arab League, on the need for a response to Syria's use of chemical weapons.

US officials conceded on Thursday they lacked conclusive evidence that Assad personally ordered last week's poison gas attack, and some allies have warned that military action without UN Security Council authorisation risks making the situation worse.

On the call with lawmakers, administration officials, including Secretary of State John Kerry and Secretary of Defence Chuck Hagel, cited evidence of chemical weapons use including “intercepted communications from high-level Syrian officials”, and seeing personnel being moved around Damascus, said Engel, the top Democrat on the House of Representatives Foreign Affairs Committee.

Engel said the vote in the British parliament was raised during the call, and “the response was that the president will do what he feels is in the best interests of the American people regardless of what other countries may or may not do”.

After the 90-minute briefing, some lawmakers said the administration still had work to do to convince the public.

“The president is going to have to make his case I think to the American people I think before he takes any action,” said Republican Howard “Buck” McKeon of California, chairman of the House Armed Services Committee.

But any intervention looked set to be delayed at least until UN investigators report back after leaving Syria on Saturday.

Syrian opposition sources said Assad's forces had removed several Scud missiles and dozens of launchers from a base north of Damascus, possibly to protect them from a Western attack, and Russia was reported to be moving ships into the region.

Expectations of imminent turmoil eased as the diplomatic process was seen playing out into next week, and the White House emphasised that any action would be “very discrete and limited”, and in no way comparable to the Iraq war.

The United States and its allies have “no smoking gun” proving Assad personally ordered the attack on a rebel-held Damascus neighbourhood in which hundreds of people were killed, US national security officials said.

Syria denies blame for the gas attacks and says they were perpetrated by rebels. Washington and its allies say the denial is not credible.

While UN chemical weapons inspectors spent a third day combing the rebel-held area where the attack took place, traffic moved normally elsewhere in Damascus, with some extra army presence but little indication of any high alert. - Reuters