Chemical weapons are being prepared by the Syrian regime, which is set to launch an offensive on the northwestern province of Idlib, U.S. special representative for Syria James Jeffrey said Thursday.

The situation in Idlib is "very dangerous" with "lots of evidence that chemical weapons are being prepared," Jeffrey told reporters in Washington.

On Aug. 21, the U.S., UK and France vowed to respond appropriately to any further use of chemical weapons by Syria.

Jeffrey said any offensive by Syrian and Russian forces in the region would be a "reckless escalation".

The Syrian regime has recently announced plans to launch a major military offensive in the area, which is controlled by various armed opposition groups.

Russian warplanes on Tuesday pounded civilian and opposition targets in Idlib.

Jeffrey said Syrian leader Bashar al-Assad "has no future as a ruler" but it was not Washington's business to get rid of him.

The U.S. will remain in Syria until Daesh is defeated, which Jeffrey said would take a while.

He added that U.S. President Donald Trump was "on board" with the commitment to remain in Syria to defeat the terror group.