US President Donald Trump has said a decision on action in Syria will be made "fairly soon".
He told reporters on Thursday that he and his team were looking "very, very seriously" at the situation, following an alleged chemical attack on the Syrian town of Douma, BBC reported.
The White House said 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.
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.
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.
He later told reporters at the White House: "We're having a meeting today on Syria... We have to make some further decisions. So they'll be made fairly soon."
Also on Thursday, US Secretary of Defence Jim Mattis told a congressional panel: "I believe there was a chemical attack and we are looking for the actual evidence."
Activists and medics say dozens of people died when government aircraft dropped bombs filled with toxic chemicals on Douma on Saturday.
President Bashar al-Assad's government denies being behind any chemical attack.
The international Organisation for the Prohibition of Chemical Weapons (OPCW) is set to send monitors to Douma to gather evidence.
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