French President Emmanuel Macron and his US counterpart Donald Trump vowed there would be "no impunity" in the event of further chemical weapons use in Syria in a telephone call Friday.
The leaders also urged Russia to put "maximum pressure" on its ally Damascus to commit to a United Nations ceasefire, warning of an increasingly dire humanitarian crisis in war-torn Syria.
Macron "stressed there will be a firm response in the case of proven use of chemical weapons leading to the death of civilians, in close coordination with our American allies," a statement from the French presidency said.
"France and the United States will not tolerate impunity."
The rising pressure on Damascus and its key ally Moscow comes after new reports last weekend of suspected chlorine use in the battered rebel-held enclave of Eastern Ghouta.
France has repeatedly warned that evidence of further use of chemical weapons in Syria is a "red line" that would prompt French strikes in a brutal seven-year conflict that has drawn in foreign powers from Russia to Turkey.
Washington has asked the UN Security Council to set up a new inquiry on chemical weapons attacks, for which the Syrian regime has repeatedly denied responsibility.
In Eastern Ghouta, hundreds of civilians have been killed since the Syrian army began new air strikes on February 18 in a bid to root out Islamist rebel groups, sparking global outrage.
A five-hour daily "pause" announced by Moscow on Monday has led to a reduction in the bombardment, but it falls far short of the UN ceasefire agreed through a Security Council resolution Saturday that has yet to be implemented.
Macron and Trump pledged to "work together to allow the resolution to be put in place to allow a ceasefire, delivery of humanitarian aide and evacuation of the injured," the French statement said.
The UN Human Rights Council is to hold an emergency meeting on Eastern Ghouta on Friday after dozens of aid trucks were unable to reach 400,000 suffering civilians a day earlier.
Aid workers have warned of dire medical needs in the enclave and the urgent need to evacuate the many sick and wounded.