Beirut - Thousands of people have been displaced and dozens killed in one of the worst flare-ups to rattle a precarious truce deal in northwestern Syria, a monitor said yesterday.

Twenty regime troops and allied fighters were killed in attacks by an extremist group on the edge of the Idlib region since Sunday, the Syrian Observatory for Human Rights said.

The latest casualties were five regime and allied fighters killed yesterday near a planned buffer zone around rebel-held territory in neighbouring Idlib.

The Britain-based Observatory said the attack was led by Hurras Al Deen, an alliance formally linked to Al Qaida that includes Syrian and foreign extremists.

At least nine extremist fighters were killed in the clashes, among the deadliest since a deal was reached in September last year to spare the region a massive regime assault.

Regime bombardment near Khan Shaikhun, in Idlib province, also killed two civilians yesterday, raising the civilian death toll to 42 since February 9, the Observatory said.

The Idlib region is mainly controlled by the extremists of Hayat Tahrir Al Sham (HTS), a Syrian group led by former Al Qaida fighters, after they pushed back smaller, Turkey-backed rebel outfits last month.

Under the September 17 deal, all fighters in the zone were supposed to withdraw their heavy weapons and extremists including HTS and Hurras Al Deen were supposed to leave.

Increased regime shelling on the Idlib town of Khan Shaikhun has sparked one of the largest waves of displacement since the truce agreement was struck in the Russian resort of Sochi.

“The UN remains deeply concerned for the safety and protection of thousands of people recently displaced from Khan Sheikhun following an increase in hostilities in the area,” David Swanson, spokesman for the UN Regional Office for the Syria Crisis in Amman told AFP.