Updated: 12/04/18 : 13:51:46
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 strike Syria.
Western states are thought to be preparing for missile strikes in response to the alleged attack.
In Russia, Syria's main military ally, 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", said the president, who has cancelled a planned trip to stay in the US with his defence secretary, and who has been canvassing support for strikes from the leaders of other countries including the UK and France.
The French leader 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.
"France will not allow any escalation that could harm the stability of the region as a whole but we can't let regimes that think they can do everything they want, including the worst things that violate international law, to act."