Media playback is unsupported on your device Media caption Defence Secretary Philip Hammond: 'Parliament has spoken' about Syria

The defence secretary has said MPs could be asked to vote again on action in Syria, but only if circumstances changed "very significantly".

Labour urged Philip Hammond to "spell out" what would change his mind, after the coalition accepted the defeat of its Commons motion last week.

But he said it was a "bit rich" of the opposition to "demand" answers, having led protests against military action.

Meanwhile, the US has delayed strikes pending the approval of Congress.

Secretary of State John Kerry now says the US has evidence the chemical nerve agent sarin was used by President Bashar al-Assad's government in a deadly attack in Damascus on 21 August.

The US put the death toll at 1,429, including 426 children.

Image caption UN weapons inspectors have now left Syria and are analysing their samples

In other developments:

A senior UN official says the Syrian refugee situation is the worst displacement crisis of all time, with seven million people estimated to have fled their homes

A BBC poll suggests nearly three-quarters of Britons believe MPs were right to reject UK military action in Syria - and two thirds did not care if it damaged US-UK relations

The Ministry of Defence confirms "no UK Armed Forces personnel, including those seconded to work with the armed forces of ally nations, will participate in... military action" against Syria

France says its intelligence indicates Syria's government carried out the attack, as pressure grows for a parliamentary vote on military action

Arab League foreign ministers call for "deterrent and necessary measures" against the Assad regime, but Lebanon and Iraq do not back the call

US ally Jordan rules out joining any coalition against Damascus

The UK government rejects newspaper claims chemicals exported from Britain could have been used in the production of chemical agents

US Senator John McCain says a vote by Congress against President Obama's proposal for military force in Syria would be catastrophic

President Assad denies his forces were behind the attack and warns military intervention would have "negative repercussions"

Last week, the government lost a Commons vote on supporting action, in principle, against Assad's government by 13 votes.

Thirty-nine Conservative and Liberal Democrat MPs sided with Labour to bring about the defeat, with several senior ministers since conceding that the question would not be asked again.

Analysis We now have a people carrier's worth of political heavyweights who didn't like the outcome of last week's Commons vote on Syria musing that perhaps Parliament could be asked to vote again. Lords Howard and Ashdown clambered on board over the weekend, as did Sir Malcolm Rifkind. Now Boris Johnson has joined them. The essence of their case is to test Labour's argument. Ed Miliband didn't rule out military action in principle, so would he change his mind if the evidence changes? But Labour can say this is irrelevant because the prime minister has made it clear the UK won't be involved. Nick Clegg has repeated that the UK will not be involved in military action in response to the use of chemical weapons in Damascus last month. But what would the UK do if there was another chemical weapons attack?

An ICM poll for the BBC - speaking to 1,000 people by telephone between Friday and Monday - suggested nearly three-quarters of people believed MPs had taken the right decision, and thought it would not harm US-UK relations.

But the US delay and its claims of fresh evidence against the Syrian regime has prompted suggestions from senior politicians that the House of Commons could reconsider its position.

Former Lib Dem and Conservative leaders Lord Ashdown and Lord Howard, and former Foreign Secretary Sir Malcolm Rifkind, have said the US's own delay could allow the House of Commons to "think again".

Conservative London Mayor Boris Johnson also insisted a new proposal "inviting British participation" could still be put before Parliament,

In the Commons, Labour's shadow defence secretary Jim Murphy asked Mr Hammond to "spell out in what, if any, circumstances" the government would hold a second parliamentary debate on UK military intervention in Syria.

Mr Hammond replied: "It's a bit rich for you, who last week trooped into the lobbies behind your leader, giving rise to the very situation we now find ourselves in, to demand I tell you precisely in which circumstances we might revisit this issue.

"I've said already we believe Parliament has spoken clearly on this issue and is unlikely to want to revisit it unless the circumstances change very significantly."

The BBC's James Landale said Labour sources had cited examples of what would need to change - such as al-Qaeda getting possession of huge stocks of chemical weapons or a direct threat to UK national security emerging - for Parliament to reconsider the matter.

'Helping al-Qaeda'

But, questioned at a press conference in London, Deputy Prime Minister Nick Clegg said: "We're not going to go back to Parliament with the same question on the same issue, in response to the same atrocity the week before last, because that decision was made by Parliament."

Media playback is unsupported on your device Media caption Deputy Prime Minister Nick Clegg says there is no point going back to Parliament over the same question

The prime minister's official spokesman also told reporters the government had "absolutely no plans to go back to Parliament".

He said the UK would continue to make the case for a robust response to President Assad at the G20 later this week.

Meanwhile, Syrian Deputy Foreign Minister Faisal Mekdad told the BBC that any military action against Syria would amount to "support for al-Qaeda and its affiliates", claiming armed groups backed by the US had used chemical weapons - not Syrian troops.

President Obama's surprise decision to ask Congress for approval means that a strike which was thought to be imminent will now not go ahead before 9 September - when Congress reconvenes - at the earliest.

The opposition Syrian National Coalition called Mr Obama's decision a "failure of leadership", saying it could "embolden" President Assad's forces.

More than 100,000 people are estimated to have died since civil conflict erupted in Syria in March 2011.

The violence began when Syrian security forces clamped down on anti-government protests.