France's government spokesman said on Tuesday that Paris would respond if it was proven that forces backing the Syrian government carried out a deadly chemical attack against the town of Douma in eastern Ghouta over the weekend.
The subject of chemical weapons' use in Syria has been a thorny issue for Macron. He has warned that use of chemical weapons to deadly effect was a "red line" that would draw French action, even unilateral.
His aides have said a military response would hinge on French intelligence proving both the use of deadly chemical agents and fatalities, and that a riposte would most likely be in coordination with the United States.
"The president has said repeated again and again that if the red line is crossed, and if it is established who is responsible, it will lead to a response," Benjamin Griveaux told Europe 1 radio.
Macron and US President Donald Trump spoke for the second time in two days on Monday after both agreeing that a chemical weapon had been used. A Syrian medical relief group said at least 60 men, women and children had been killed.
After an initial cautious statement Sunday night, Macron's office said on Tuesday that the two had agreed on the need for a "strong reaction" from the international community.
Trump on Monday promised quick, forceful action. French officials said they were coordinating the response with their allies and that all options were being studied.
According to the defense ministry, France has a dozen Rafale fighter jets within striking distance positioned at bases in Jordan and the United Arab Emirates.
It could also opt to launch raids from France itself and has a frigate in the Mediterranean that could fire cruise missiles.