The US has forcefully condemned what it now says was an "undeniable" chemical attack in Syria, raising the possibility of military intervention.
Secretary of State John Kerry stressed the importance of accountability for any party that has used chemical weapons, but added that President Obama has not yet decided on a response.
It comes as UN inspectors were granted access to the sites of the alleged chemical attack in Damascus, where they spoke to witnesses and collected samples.
Prime Minister David Cameron will return to Downing Street tomorrow ahead of a meeting of the National Security Council on Wednesday.
Some members of the UN inspection team came under sniper fire as they drove to the sites of the allaeged attack today.
The gunfire delayed the fact-finding mission but did not stop it. UN Secretary General Ban Ki-Moon later said the UN would register a "strong conplaint" to both the Syrian government and opposition.
Secretary Kerry ackowledged that the UN mission was ongoing, but described the evidence that already exists as "real and compelling".
A spokeswoman for the State Department later confirmed that the US government has "very little doubt" that responsibility for the alleged chemical attack lay with the Syrian regime.
The ratcheting up of rhetoric was also reflected in the UK, where Foreign Secretary William Hague told the BBC that a British response to the crisis would not be constrained by the divisions in the UN Security Council.
– William Hague, Foreign Secretary
Is it possible to respond to chemical weapons without complete unity on the U.N. Security Council?
I would argue yes it is otherwise it might be impossible to respond to such outrages, such crimes, and I don't think that's an acceptable situation.
The Prime Minister and Deputy Prime Minister are both cutting short pre-planned trips in order to discuss a response to the crisis.
There is also the possibility that MPs could be recalled from parliamentary recess to vote on a response to Syria, although this will not be decided until Tuesday at the earliest.
The Prime Minister has already been discussing the situation with world leaders, reportedly clashing with Russia's President Putin.
He told Mr Putin there was "little doubt" that the government was behind the alleged atrocity and "no evidence" to back the regime's claims that opposition forces were responsible, a Downing Street spokesman said.
The Russian president disagreed, saying that there was "no evidence of whether a chemical weapons attack had taken place or who was responsible".
A chemical attack is reported to have killed up to 1,300 people in several suburbs in the east of the Syrian capital Damascus in the early hours of 21 August.
Videos and accounts circulated on social media tell of an unusual smell in the air and of symptoms consistent with a chemical attack.