There is “lots of evidence” chemical weapons are being prepared by Syrian government forces in Idlib, north-west Syria, the new US representative for Syria has said, warning any attack on the last big rebel enclave would be a “reckless escalation”.

“I am very sure that we have very, very good grounds to be making these warnings,” said Jim Jeffrey, who was named on 17 August as secretary of state Mike Pompeo’s special adviser on Syria overseeing talks on a political transition. “Any offensive is to us objectionable as a reckless escalation,” Jeffrey said. “There is lots of evidence that chemical weapons are being prepared.”

Washington has issued a strong warning to Syria’s government against using chemical weapons in the widely expected operation.

Russian-led advance on Idlib could displace 700,000 Syrians Read more

Jeffrey said any offensive by Russian and Syrian forces, and the use of chemical weapons, would force huge refugee flows into south-eastern Turkey or areas in Syria under Turkish control.

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

The fate of the insurgent stronghold in and around Idlib province now seems to rest 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.

Backed by Russian air power, Assad has over several years taken back one rebel enclave after another. Idlib and its surroundings are now the only significant area where armed opposition to Damascus remains.

Jeffrey described the situation in Idlib as “very dangerous” and said Turkey, which has backed some rebel groups in the region, was trying to avoid an all-out Syrian government offensive.

“I think the last chapter of the Idlib story has not been written. The Turks are trying to find a way out. The Turks have shown a great deal of resistance to an attack,” he said.

He said the US had repeatedly asked Russia whether it could “operate” in Idlib to eliminate the last holdouts of Isis and other extremist groups. Asked whether that would include US air strikes, Jeffrey said: “That would be one way.”