Syria crisis: David Cameron supports Obama's stance
David Cameron has given his support for Barack Obama over Syria after the US president said he would ask Congress to vote on military strike action.
In a tweet, the prime minister said: "I understand and support Barack Obama's position on #Syria."
Former Lib Dem leader Lord Ashdown said the UK could now "reconsider" its decision to not intervene.
It comes after MPs blocked a motion to support military action if backed by evidence from UN weapons inspectors.
The defeat of the government's motion on Thursday came as a surprise and led Chancellor George Osborne to say there would now be "national soul searching" about Britain's role in the world.
The prime minister had pushed for military action following the suspected chemical weapons attack on 21 August. The US says it killed 1,429 people and was carried out by the Syrian government.
It is a claim that Syrian President Bashar al-Assad denies.
President Obama had previously said that the use of chemical weapons would represent a "red line" that would change his thinking about intervening in a conflict in which more than 100,000 people have died.
UN inspectors investigating the attacks arrived in the Netherlands on Saturday with samples from site visits, which will be tested in laboratories in Europe.
President Obama said in his speech on Saturday that he had decided the US should take military action against the Syrian regime, but said he would formally ask congress to authorise military action.
He said he was "confident" that the US government had made a case for military action without the need to wait for the United Nations inspectors to compile their report.
He added he was comfortable going forward without the support of the UN Security Council "that, so far, has been completely paralysed and unwilling to hold Assad accountable.
"As a consequence, many people have advised against taking this decision to Congress, and undoubtedly they were impacted by what we saw happen in the United Kingdom this week when the Parliament of our closest ally failed to pass a resolution with a similar goal, even as the prime minister supported taking action.
"Yet, while I believe I have the authority to carry out this action without specific congressional authorisation I know that the country will be stronger if we take this course and our actions will be even more effective."
UK Foreign Secretary William Hague tweeted that President Obama's address was a "fine speech".
And Lord Ashdown said: "This was a brave and principled act from a brave and principled president.
"It opens up all sorts of different new possibilities, one of which is the UK Parliament certainly could reconsider its position."
Lord Ashdown had been critical of the decision to not take military action in Syria in the aftermath of the vote, saying the UK was "hugely diminished".
Writing in the Observer on Sunday, he said the result of Thursday's vote had "made it more difficult for Barack Obama to act".
Labour leader Ed Miliband has not commented on the president's speech, but has previously said he does not believe that the House of Commons vote means the UK cannot make a difference to "innocent civilians" in Syria.
'Pause for thought'
The White House has now sent draft legislation to Congress formally asking for approval to use military force in Syria to "deter, disrupt, prevent and degrade" the potential for further chemical attacks.
Although the president acknowledged that US military chiefs stood ready to act, his pledge to wait for a vote effectively rules out intervention until Congress returns to session on 9 September.
Russian President Vladimir Putin - an ally of Syria - has challenged the US to present its case for military intervention to the UN, and has called for further talks at the G20 summit in St Petersburg on Thursday.
Russia has previously warned that "any unilateral military action bypassing the UN Security Council" would be a "direct violation of international law".
The five permanent members of the UN Security Council - Russia, China, the US, UK and France - have been divided on taking action in Syria.
Meanwhile, former UK Prime Minister Tony Blair said the Commons vote - although "shocking" - was "understandable" given that intervention in Iraq and Afghanistan had been "far more perilous and difficult than we expected".
People fear that "we will end up in a 'quagmire'" in Syria, he said in the Sunday Times.
But he added the conflict in Syria was "crucial to our security" and warned that Britain would have to take sides sooner or later.
"Intervention can be uncertain, expensive and bloody. But history has taught us that inaction can merely postpone the reckoning," he said.
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.
President Assad blames opposition forces for last week's attack and has said his country will defend itself against any Western "aggression".
Forces which could be used against Syria:
•Four US destroyers - USS Gravely, USS Ramage, USS Barry and USS Mahan - are in the eastern Mediterranean, equipped with cruise missiles. The missiles can also be fired from submarines, but the US Navy does not reveal their locations
•Airbases at Incirlik and Izmir in Turkey, and in Jordan, could be used to carry out strikes
•Two aircraft carriers - USS Nimitz and USS Harry S Truman are in the wider region
•French aircraft carrier Charles de Gaulle is currently in Toulon in the western Mediterranean
•French Rafale and Mirage aircraft can also operate from Al-Dhahra airbase in the UAE