Western countries are increasingly convinced that Syrian government forces have used chemical weapons - and that they should respond with military action.
For its part, Syria continues to deny the claims - and is demanding that its accusers produce evidence.
The White House claims the use of chemical weapons in Syria is "undeniable" and a "moral obscenity".
White House spokesman Jay Carney says punitive strikes against Syrian government forces are now being considered by President Barack Obama.
He says not to act would be to invite further use of chemical weapons.
But he denies any foreign intervention would be about regime change.
"They are about responding to a clear violation of an international standard that prohibits the use of chemical weapons. We are also very much engaged in an effort to support the opposition in its struggle with the Assad regime as the Assad regime continues to try to massacre its own people in an effort to maintain power and it is our firm conviction that Syria's future cannot include Assad in power. But this deliberation and the actions we are contemplating are not about regime change."
Britain and France are also speaking in similar terms.
Speaking with Dutch national TV, the British Prime Minister David Cameron says any international strikes against Syria must be used to prevent more chemical weapons attacks.
"And of course any action we take or others take would have to be legal, would have to be proportioned, it would have to be specifically to deter and degrade the future use of chemical weapons. Let me stress to people this is not about getting involved in a Middle Eastern war or changing our starts in Syria or going further into that conflict. It's nothing to do with that. It's about chemical weapons, their use is wrong and the world shouldn't stand lightly by."
Australia's Prime Minister Kevin Rudd agrees there's now overwhelming evidence of chemical attacks in Syria, which, he says, would be tantamount to crimes against humanity.
He says the Australian government has high confidence that Syrian government forces were responsible for the strikes near Damascus which reportedly killed more than 300 people.
Mr Rudd has not specified which military options are on the table, but the Foreign Minister Bob Carr says a re-elected Labor government wouldn't send Australian troops to Syria in the event of Western military intervention.
The Syrian government, meanwhile, continues to deny the use of chemical weapons, and is vowing to resist any military action against it.
Foreign Minister Walid al Muallem says the work of United Nations inspectors is being facilitated, but they won't find any evidence of government use of chemical weapons.
He says it would be a mistake for the United States and its allies to launch any military action against Syrian targets in retaliation for alleged use of the weapons.
"We are all hearing the drums of war being beaten around us. If they're willing to launch an aggression or military act against Syria, I believe the pretext of chemical weapons is false, baseless and groundless. And I challenge, I dare them, to produce any single piece of evidence."
Russia is accusing Western countries of creating unsubstantiated allegations against the Syrian government, to try to justify military intervention.
Russian Foreign Ministry spokesman Alexander Lukashevich has warned the US against any military action without United Nations Security Council authorisation.
"The attempts to get around the Security Council and to once again create artificial unsubstantiated pretexts for the armed interference in the region may result in new sufferings in Syria and in catastrophic consequences for other countries of the Middle East and Northern Africa. We call on our American colleagues and all members of the world community to be sensible, to strictly observe international law and first of all the basic principles of the United Nations Charter."
But US Defence Secretary Chuck Hagel has told the BBC, the American military is already prepared to act if President Barack Obama gives the order.
"We are prepared, we have moved assets in place to be able to fulfil and comply with whatever option the president wishes to take, if he wishes to take any of the options that he's asked for. ... We are ready to go."
There's speculation that military action by the US, Britain and France could involve attacks on selected targets in Syria from warships or submarines.
The US has airbases in Turkey and Jordan capable of launching strikes against Syria, whilst cruise missiles could also be fired from US ships and submarines in the Mediterranean.
Britain could send missiles into Syria from its military airbase in Cyprus.
