France's top diplomat said on Tuesday that the country had an “indication” that a chemical attack had been carried out in Syria's Idlib province this month, an attack alleged by Washington last week.

“We have an indication that chemical weapons were used in the Idlib region, but for now it has not been verified,” Foreign Minister Jean-Yves Le Drian told a parliamentary commission.

“We're being cautious because we consider that chemical weapons use has to be proven and be lethal, in which case we can react,” he said.

President Emmanuel Macron has made use of chemical weapons by the Syrian government a “red line” that would trigger a military reprisal against the regime of President Bashar al-Assad.

The United States has also threatened reprisals if the suspected Idlib attack is proven.

Russia and Turkey reached an agreement last September that nominally protects Idlib amid fears for the safety of some three million people in the northwestern area of Syria.

But Syria's former Al Qaeda affiliate, Hayat Tahrir al-Sham, has seized a large part of the province and adjoining areas, triggering a regime offensive that includes strikes by Syrian and Russian airplanes.

International inspectors say that Assad's forces have carried out a series of chemical attacks in the course of the brutal civil war, which has killed more than 370,000 people and displaced millions since 2011.

A sarin gas attack in April 2017 in the town of Khan Sheikhun killed 83 people, according to the United Nations, leading US President Donald Trump to order a strike by 59 cruise missiles on a Syrian air base.

And France joined Britain and the US in launching missile strikes on three suspected chemical weapons sites in Syria in April 2018 after a suspected chemical attack in Douma.