France's President Emmanuel Macron has "proof" the Syrian government used chemical weapons on Douma last weekend and will decide in due course about air strikes, he says.
The French leader had previously said any strikes would target the Syrian government's "chemical capabilities".
MSNBC also reported US officials had obtained blood and urine samples from victims of the alleged chemical attack in Syria last weekend and the samples tested positive for chemicals, mainly for chlorine and some for a nerve agent.
The Russian Ambassador to the UN urged the US and its allies to refrain from military action against Syria.
He said the Syria situation was "very dangerous" and the immediate priority would be to "avert the possibility of war".
Asked in a TV interview whether France would join strikes on Syria, Mr Macron said: "We will need to take decisions in due course, when we judge it most useful and effective.
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."
"Regimes that think they can do everything they want, including the worst things that violate international law, cannot be allowed to act."
Western states are thought to be preparing missile strikes in response to the alleged attack.
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.
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."
In Russia, Syria's main military ally, Kremlin spokesman Dmitry Peskov urged against "any steps which could lead to an escalation of tensions".
Russia has described the reports of a chemical attack as a "provocation" designed to justify Western intervention against the Syrian government, and accused militant rebels of fabricating it.
Senior Russian figures have warned that missiles threatening the country's forces in Syria will be shot down and their launch sites targeted.
On Thursday, a foreign ministry spokeswoman asked whether the aim of Western strikes might be "to quickly remove the traces of the provocation".
President Vladimir Putin said he hoped common sense would prevail and that the situation would stabilise.
Attack on Douma
Douma was the last major rebel stronghold near Damascus. Local activists say the main leaders of the group that held it have left, following an agreement between Russia and the rebels.
The attack on the city on Saturday is said by activists and medics to have killed dozens of people when government aircraft dropped bombs filled with toxic chemicals.
The international Organisation for the Prohibition of Chemical Weapons (OPCW) is due to send monitors to Douma, but it is how much evidence of any chemical attack might remain.
Mr Assad's government has denies being behind any chemical attack.
Russian claims that the town had been taken over by Syrian forces could not be independently verified.'
- BBC / Reuters