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.