Chemical weapons 'being prepared' in Syria: US envoy
On Aug. 21, the U.S., UK and France vowed to respond appropriately to any further use of chemical weapons by Syria.
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 Sept. 4 pounded civilian and opposition targets in Idlib.
The U.S. will remain in Syria until the Islamic State of Iraq and the Levant (ISIL) group 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 ISIL.