Macron said one of his aims in Syria was to "remove the regime's chemical attack capabilities", but added that he wanted to avoid "an escalation".
He said the Syria situation was "very dangerous" and the immediate priority would be to "avert the possibility of war".
Both the Syrian regime and its main global backers Russian Federation and Iran have denied responsibility for the alleged chemical attack and insist "there is a Western conspiracy" aimed at justifying military action in Syria.
French President, Emmanuel Macron said, "We have proof that last week, now almost 10 days ago, that chemical weapons were used, at least chlorine, and that they were used by Bashar al-Assad's regime".
"Regimes that think they can do everything they want, including the worst things that violate worldwide law, cannot be allowed to act".
Moscow, which offers military support to the Syrian regime, has warned of a possible Russian response to a USA attack. Also on Thursday, the UN Security Council was to hold an emergency meeting to discuss the crisis, the BBC reported.
French and United States officials were "working very closely, and we will have decisions to take, at the time we choose, when we consider it most useful and most effective". Trump tweeted Thursday: "Never said when an attack on Syria would take place".
Kremlin spokesman Dmitry Peskov urged against "any steps which could lead to an escalation of tensions".
It was the first public acknowledgement by Jaish al-Islam of a deal reached for Douma, their last rebel holdout in the Eastern Ghouta suburb of Damascus.
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.
Syria and its ally Russian Federation have both denied involvement amid worldwide outrage over Saturday's incident.
Macron did not describe the evidence or say how France got it.