The new US envoy for Syria has said there is “lots of evidence” that Syrian government forces are preparing to use chemical weapons in Idlib.
Jim Jeffrey said an anticipated offensive against the last major rebel-held area would be a “reckless escalation”, Reuters reported.
The Syrian government has repeatedly denied ever using chemical weapons.
Russian planes have bombed rebel areas in the north-western region as troops mass for the expected offensive.
It comes as a summit is due to take place in Iran on Friday between the leaders of Russia and Iran, which both support Syrian President Bashar al-Assad, and Turkey, which has backed some rebel factions.
The UN has warned of a humanitarian catastrophe if an all-out assault takes place and Turkey fears a new influx of refugees across its border.
“I am very sure that we have very, very good grounds to be making these warnings,” Mr Jeffrey said in his first interview since being appointed.
“Any offensive is to us objectionable as a reckless escalation. There is lots of evidence that chemical weapons are being prepared.”
He did not give details of the evidence he was referring to.
