"All indications... tell us today that chlorine is being used by the regime at present in Syria," Foreign Minister Jean-Yves Le Drian told BFM television.

"I'm weighing my words because as long as we haven't completely documented this we have to stay prudent," he said.

UN war crimes investigators said Tuesday that they were studying "multiple" reports that chemical weapons have been used in the rebel-held zones of Eastern Ghouta, near Damascus, and in the northwestern Idlib Province in recent weeks.

The United States said this week there was "obvious evidence" of recent chlorine gas attacks, including in Eastern Ghouta.

Asked how France would respond, Le Drian pointed to the "partnership against impunity" agreed by two dozen countries in January to ensure that perpetrators of chemical attacks in Syria are held accountable.

But he did not allude to any other response, including military retaliation, that France might take against the regime of President Bashar Al-Assad if the attacks are confirmed.

Shortly after taking office last year, French President Emmanuel Macron said chemical attacks in Syria would be a "red line" for France.

Le Drian also said Kurdish forces fighting the Islamic State group in northeast Syria had detained "about 100" French jihadists.

"We're being told that about 100 have been arrested by the Kurds in Syria," he said.

French officials have shown little inclination to repatriate any of these captured fighters, as well as those facing potential death penalties in Iraq, saying they should face local courts.