UNITED NATIONS — U.N. Secretary-General Antonio Guterres is warning of the growing risk of a humanitarian catastrophe in rebel-held Idlib province in Syria if there's a full-scale military operation.

U.N. spokesman Stephane Dujarric said Wednesday that Guterres urgently appeals to the Syrian government and opposition forces to exercise restraint and make the protection of civilians a priority.

He said Guterres calls on Russia, Iran and Turkey as guarantor states trying to end the violence in Syria to find a peaceful solution in Idlib.

The Syrian government is gearing up for an expected offensive in Idlib, home to nearly 3 million people. The U.S., Britain and France have warned they will respond to any use of chemical weapons in Syria.

Dujarric said Guterres "reaffirms that any use of chemical weapons is totally unacceptable."