There is "lots of evidence" that chemical weapons are being prepared by Syrian government forces in Idlib in northwest Syria, the new U.S. adviser for Syria said on Thursday as he warned of the risks of an offensive on the country's last big rebel enclave.

"I am very sure that we have very, very good grounds to be making these warnings," said Jim Jeffrey, who was named on Aug. 17 as Secretary of State Mike Pompeo's special adviser on Syria overseeing talks on a political transition in that country.

"Any offensive is to us objectionable as a reckless escalation," Jeffrey told a few reporters in his first interview on the situation in Syria since his appointment. "There is lots of evidence that chemical weapons are being prepared."