US President Donald Trump says a decision on action in Syria will be made “fairly soon”.
He told reporters that he and his team were looking “very, very seriously” at the situation, following an alleged chemical attack on the town of Douma.
The White House said Mr Trump would talk to British and French leaders later on Thursday. Western powers are thought to be preparing for strikes.
Russia, Syria’s main military ally, strongly opposes such action.
Russia’s envoy to the UN, Vassily Nebenzia, said he “cannot exclude” the possibility of a war between Russia and the US.
“The immediate priority is to avert the danger of war,” he told reporters.
Also on Thursday, French President Emmanuel Macron said he had “proof” that the Syrian government had attacked Douma with chemical weapons.
In the UK, cabinet ministers agreed “on the need to take action” in Syria to “deter the further use of chemical weapons”, the office of Prime Minister Theresa May said.
The UN Security Council is to hold an emergency meeting later to discuss the crisis.
What has Trump said about the attack?
On Sunday, the day after the attack, the US president said Russian President Vladimir Putin bore responsibility for the “atrocity” in rebel-held Douma, because of his support for the Syrian government.
Mr Trump, who has cancelled a planned trip abroad, has been canvassing support for strikes from the leaders of France and the UK.
On Wednesday he said the missiles were “coming”, but on Thursday he tweeted that he had “never said when”. It “could be very soon or not so soon at all”, he said.