France's President Emmanuel Macron says he has "proof" that the Syrian government attacked the town of Douma with chemical weapons last weekend.
He said he would decide "in due course" whether to respond with air strikes.
Urine and blood samples from victims of the attack have tested positive for chlorine and a nerve agent, media reports quote US officials as saying.
Western states are thought to be preparing for missile strikes. Russia strongly opposes such action.
Kremlin spokesman Dmitry Peskov urged against "any steps which could lead to an escalation of tensions".
US President Donald Trump, who said on Wednesday that missiles were "coming", has now tweeted that he "never said when".
It "could be very soon or not so soon at all", tweeted the president, who has cancelled a planned trip to allow him to stay in the US with his defence secretary, and has been canvassing support for strikes from the leaders of France and the UK.
Also on Thursday, US Secretary of Defense Jim Mattis told a congressional panel: "I believe there was a chemical attack and we are looking for the actual evidence."
Nato Secretary General Jens Stoltenberg said: "We condemn in the strongest terms the use of chemical weapons. The latest attack was horrendous."
The British Cabinet is meeting to discuss the government's response. Later in the day, the UN Security Council will hold an emergency meeting on the crisis.
The French leader had previously said any strikes would target the Syrian government's "chemical capabilities".
He did not give the source of his information but said: "We have proof that last week chemical weapons, at least chlorine, were used by the regime of Bashar al-Assad."
Asked in a TV interview whether France would join strikes on Syria, he said: "We will need to take decisions in due course, when we judge it most useful and effective.
"Regimes that think they can do everything they want, including the worst things that violate international law, cannot be allowed to act."
