France has repeatedly warned that evidence of further use of chemical weapons in Syria was a “red line” that would prompt French strikes.
“The use of chemical weapons is a war crime,” Le Drian said in a statement, adding that France had asked the UN Security Council to convene as soon as possible to examine the situation in Eastern Ghouta.
The alleged chemical attack that left dozens dead in Syria's rebel-held town of Douma sparked international outrage Sunday, with US President Donald Trump warning there would be a “big price to pay”.
Douma is the last remaining opposition-held town in Ghouta, once the rebels' main bastion outside Damascus but now ravaged by a seven-week regime assault.
In March, French President Emmanuel Macron and US President Donald Trump vowed there would be “no impunity” in the event of further chemical weapons use in the conflict.
The government of Syrian President Bashar Al-Assad and Moscow have both denied any use of chemical weapons as “fabrications”.
“France is working with its allies and international organisations to verify the reality and the nature of these strikes,” Le Drian added, calling the bombings “a flagrant violation of international humanitarian law”.
Hundreds of civilians have been killed in Eastern Ghouta since the Syrian army began new air strikes on February 18.
Aid workers have warned of dire medical needs in the enclave and the urgent need to evacuate the many sick and wounded.