Airstrikes have been stepped up against rebel-held northwestern Syria, the last major bastion of opponents of President Bashar Assad, prompting thousands of civilians to flee the town of Khan Sheikhoun, war monitors said on Tuesday.

The area near the Turkish frontier is under a cease-fire brokered last year by Assad's main battlefield ally, Russia, and Turkey, one of the main supporters of rebels who have fought to topple him for eight years.

Hundreds of thousands of people who fled other parts of Syria are sheltering in the enclave, protected by the truce, which was aimed at averting an all-out government assault. Any suggestion that the cease-fire is under new strain is closely watched by the warring parties and their allies.