The Syrian government has said it will allow UN weapons inspectors to examine the site of an alleged chemical weapons attack near Damascus.
A team of experts is expected to begin an investigation tomorrow after Bashar Assad's regime pledged to maintain a ceasefire at the site in Damascus.
However, Washington has accused the government of having "something to hide" and delaying access for four days to cover up evidence.
An intense round of diplomacy on the crisis has also been continuing, with David Cameron and French President Francois Hollande insisting the "crime must not be swept under the carpet".
A Number 10 spokesman said the leaders had both expressed the view during a telephone call that "the failure of the regime to grant UN investigators immediate access to the area was extremely worrying and demonstrated that the regime was hiding the facts and evidence".
"They agreed that a chemical weapons attack against the Syrian people on the scale that was emerging demanded a firm response from the international community," the spokesman added.
Last night the Prime Minister and Barack Obama spent 40 minutes discussing the situation on the telephone, and ordered officials to examine "all options" for a response.
Obama previously suggested that the use of chemical weapons in Syria would be a "gamechanger".
But he has stopped short of committing to direct intervention in the troubled country - a step that would risk inflaming tensions with Russia.
Cameron would also face domestic resistance to military action, with Tory MPs already having insisted there should be a Commons vote before arms are supplied to rebels.
But there is speculation that the US, Britain and France could back limited airstrikes to demonstrate that deployment of chemical weapons will not be tolerated.
Medecins Sans Frontieres said hospitals it supports in Syria treated some 3,600 patients with "neurotoxic symptoms" on Wednesday, 355 of whom died.
It said the patients had arrived in three hospitals in the Damascus area. Staff described people suffering from convulsions, extreme salivation, contracted pupils and sight and respiratory problems.
However, the organisation stressed it could not "scientifically confirm" the use of chemical weapons.
Syrian state media accused rebel forces of using chemical agents, saying some government soldiers had suffocated as a result during fighting in the suburb of Jobar.
Information minister Omran Zoabi also warned the US and allies that military action would not be a "walk in the park".
"If the US leads a military intervention, this will have dangerous consequences. It will bring chaos and the region will burn," he said.
Former Liberal Democrat leader Lord Ashdown said the evidence of chemical weapon deployment degraded very quickly, but insisted he hoped proof could be found even now.
Speaking on BBC Radio 4's World This Weekend, he urged Foreign Secretary William Hague against rushing to condemn the attack before the UN experts had examined the area.
Shadow foreign secretary Douglas Alexander said: "In light of ongoing international developments, it is right that the UK works with the international community to try and find an agreed way forward in Syria.
"If, in reality, the Prime Minister is now considering military options involving UK personnel then of course I would expect him to seek a recall of Parliament and to come to the House of Commons and make his case in advance of a decision being made."
A Downing Street source played down speculation over a recall of Parliament, insisting the timetable for any response by the international community was not yet clear.
They also stressed that while the prime minister had promised MPs the ability to debate such issues, the Government "has to reserve the right to act immediately".