According to Reuters, 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.”

The White House has warned that the United States and its allies would respond “swiftly and vigorously” if government forces used chemical weapons in the widely expected offensive.

Jeffrey said an attack by Russian and Syrian forces, and the use of chemical weapons, would force huge refugee flows into southeastern Turkey or areas in Syria under Turkish control.

Syrian President Bashar al-Assad has massed his army and allied forces on the frontlines in the northwest, and Russian planes have joined his bombardment of rebels there, in a prelude to a possible assault.

The fate of the insurgent stronghold in and around Idlib province rests on a meeting to be held in Tehran on Friday between the leaders of Assad’s supporters Russia and Iran, and the rebels’ ally, Turkey.