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”.
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.”
Later in the day, the British Cabinet is due to discuss its response and the UN Security Council will hold an emergency meeting on the crisis.
But Macron hasn’t shown any evidence. 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.”
Macron – who likes to be seen as a president who does what he says he will do – is facing the test of what “reprisal” actually means.
Former US President Barak Obama was derided in France for not following through on his own “red line” after Syrian attacks five years ago.
“When you fix red lines,” Mr Macron told a French newspaper last year, “if you can’t enforce them, you decide to be weak.”
Strength is an important part of President Macron’s bid to project French influence on the world stage – and a key element, too, of his new political friendship with his US counterpart Donald Trump.
Russia has described the reports of a chemical attack as a “provocation” designed to justify Western intervention against its ally, and accused militant rebels of fabricating it.
Senior Russian figures have warned that missiles threatening the country’s forces in Syria would 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.