Macron said France has "proof that last week chemical weapons were used, at least chlorine, and that they were used by Bashar al-Assad's regime". He didn't elaborate on exactly what that proof is.
France is expected to join the United States and Britain in carrying out air strikes in response to the use of the weapons but it remains unclear if it will definitely happen and if so, when.
The French leader, however, said on Thursday he would decide "in due course" whether to strike Syria.
Kremlin spokesman Dmitry Peskov cautioned against "any steps which could lead to an escalation of tensions".More news: IAG Group Gets Stake of Norwegian Air Shuttle, Prepares Buyout Offer
However, he added, France could not give "regimes who think they can do anything" free rein to violate worldwide laws and the global ban on the use of chemical weapons.
It comes as the British Prime Minister Theresa May holds a special "war cabinet" to discuss her options.
The World Health Organization (WHO) has said 500 patients were treated for symptoms of exposure to toxic chemicals.
It remains hard to place an exact figure on the number of people killed and wounded Saturday in the rebel-held town of Douma, on the outskirts of Damascus.More news: Trump's personal attorney tells ABC News FBI raids were 'respectful' and 'courteous'
The Jaish al-Islam group in Douma agreed on Sunday to withdraw, hours after the suspected chemical weapons attack on the town that has raised the prospect of US strikes.
Meanwhile, an global chemical weapons watchdog organization said Thursday that its team is en route to Syria and will begin work Saturday to investigate the attack, the Organization for the Prohibition of Chemical Weapons said.
President Assad's forces deny involvement in the attack in Douma.
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.More news: Police Arrest 2 Men In Connection With Saturday Homicide
Both Syria and Russian Federation, its biggest supporter, have denied involvement in the attack, instead claiming that rebel groups had fabricated it to thwart the advances of Syrian troops and provoke global military intervention.