Image caption Government and opposition forces have been fighting for more than two years

The UK government has ruled out a re-run of the vote on military action in Syria, the foreign secretary has said.

William Hague said ministers could not "go back every week" to something MPs rejected. It came after the US delayed strikes until Congress approves them.

The US now says it has evidence the chemical nerve agent sarin was used in a deadly attack in Damascus last month.

On Thursday, MPs voted against the principle of taking military action over the suspected attack.

They blocked the motion to support military action if backed by evidence from UN weapons inspectors.

The government lost by 13 votes after almost 40 MPs from the two coalition parties joined Labour in siding against the motion.

Some senior politicians, including former Westminster party leaders Lord Ashdown and Lord Howard and former Foreign Secretary Sir Malcolm Rifkind, have said the US delay - pending Congressional approval for strikes - could allow the House of Commons to "think again".

'Think again'

Since then, US Secretary of State John Kerry has said samples from hair and blood gathered after the attack on 21 August - which the US says was carried out by the regime - "tested positive for signatures of sarin".

"So this case is building and this case will build," he said.

But 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 America had used chemical weapons - not Syrian troops.

On Monday, London Mayor Boris Johnson became the latest senior figure to suggest that British forces could still be deployed.

Writing in the Daily Telegraph, Mr Johnson said: "If there is new and better evidence that inculpates [Syrian President] Assad, I see no reason why the government should not lay a new motion before Parliament, inviting British participation."

Analysis Spotting the likely pause before any military action as a result of President Obama consulting Congress, three Westminster big beasts each independently floated an idea. Two were former Westminster party leaders, Lord Ashdown and Lord Howard. The third was a former Foreign Secretary and Defence Secretary, Sir Malcolm Rifkind, who now chairs the Intelligence and Security Committee. Their basic pitch was this: If the facts change on Syria, could the Commons be asked to change its mind? But senior figures from both the government and Labour stressed that parliament has spoken. A second vote, with the leadership in support of military action, could split Labour down the middle. A second defeat could cost David Cameron his job.

In other developments:

Arab League foreign ministers urged the world community to "take the deterrent and necessary measures" against Syria. But several members - including Lebanon and Iraq - did not back the call

Jordan - a key US ally in the region - ruled out joining any US-led coalition against Damascus

French PM Jean-Marc Ayrault is due to meet parliamentary leaders to brief them on possible strikes. Paris earlier pledged to support America's action

US lawmakers are due to reconvene next week, and White House officials have said they believe they will support the president. Democrats and Republicans have received a special intelligence briefing designed to bolster President Obama's case

UN experts are analysing samples of evidence gathered in Syria to determine whether chemical weapons attacks have taken place on a number of occasions

The UK government has rejected newspaper claims chemicals exported from Britain after the uprising in Syria began could have been used in the production of agents such as sarin gas

On Sunday, Mr Hague told the BBC he did not believe new information about the attack would make a difference to the MPs who doubted the government's case.

He said: "I don't think on any issue the government can go back to parliament every few days, or every week with the same proposition, and our proposition already included waiting for the UN weapon inspectors to report, to discuss things at the Security Council, that was already built into our proposal.

"So on this particular issue that we voted for on Thursday; can we go back in the coming days and have that vote again, well no. We can't do that. Parliament has spoken."

Earlier, Chancellor George Osborne also said that waiting for more evidence would not have made a difference to the MPs' decision.

Image caption The war inside Syria continues to cause huge damage

"They were sceptical of another foreign entanglement," he told the BBC's Andrew Marr Show.

"I understand their argument, I don't agree with it, and I don't feel frankly more evidence or another week or more UN reports would have convinced them."

Mr Osborne denied that he and Prime Minister David Cameron had made a miscalculation by taking the vote to Parliament.

But shadow defence secretary Jim Murphy told Sky News: "Syria is complicated, the region is complicated and there's no military exclusive solution. It's about diplomacy, it's about development policy and it is, for some countries, about deployment of military force."

Media playback is unsupported on your device Media caption George Osborne: "Very sad if we turned our back on world"

He added that if al-Qaeda was to get its hands on chemical weapons or "if there were to be really significant developments in Syria and the conditions that we set in our motion on Thursday about it being legal, about the evidence being available, compelling evidence, about a UN process, then of course the prime minister has the right to bring that back to Parliament".

Shadow foreign secretary Douglas Alexander said if Mr Cameron returned to Parliament for a second vote following Congress's decision, he would be going back on promises he made after Thursday's debate.

Sir Malcolm Rifkind said: "Parliament ought to have the opportunity to debate the matter again" if the evidence was agreed to be compelling.

The Archbishop of York Dr John Sentamu told the Andrew Marr Show he did not think it was "wise" to take military action in Syria, saying it was important to wait for the weapons inspectors' report before acting.

Sarin claim

Mr Cameron had pushed for military action following the suspected chemical weapons attack on the outskirts of the Syrian capital Damascus.

Syria's President Bashar al-Assad blames opposition forces and says his country will defend itself against any Western "aggression".

Media playback is unsupported on your device Media caption Douglas Alexander says the UK-US relationship is "strong, deep and I believe will endure"

President Obama's surprising decision to ask Congress for approval means that a strike that was thought to be imminent will now not go ahead before 9 September, when Congress reconvenes.

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

More than 100,000 people are estimated to have died and at least 1.7 million refugees displaced since civil conflict erupted in Syria in March 2011.

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