Beirut - An extremist faction in the Idlib region of northwestern Syria has rejected a Turkish-Russian deal that requires “radical” insurgents to withdraw from a demilitarised zone, and urged rebels to launch new military operations.

While the Huras Al Deen faction is not the main Islamist militant group in Idlib, its statement points to objections that may complicate the implementation of the agreement clinched last week by Russia and Turkey.

The most powerful extremist group in the northwest, Tahrir Al Sham, has yet to declare its stance on the deal, under which insurgents must vacate the demilitarised zone by October 15.

Tahrir Al Sham is a coalition of Islamist groups dominated by the faction formerly known as the Nusra Front, which was an official wing of the Al Qaida network until 2016. Tahrir Al Sham’s position will be critical.

An alliance of Turkey-allied rebel groups, the National Front for Liberation, has declared its “complete cooperation” with the Turkish effort, but has also ruled out disarming or yielding territory.

The Huras Al Deen group was formed earlier this year by combatants who split from Tahrir Al Sham and the Nusra Front when it cut its ties with Al Qaida. It includes foreign militants.

“We advise our mujahideen brothers in this decisive and dangerous phase ... (to) begin military operations against the enemies of religion to thwart their plans,” it said in a statement released on Saturday.

The demilitarised zone agreed by Turkey and Russia will be 15 to 20 km deep and run along the contact line between rebel and regime fighters. It will be patrolled by Turkish and Russian forces.

Russian President Vladimir Putin has said that all opposition heavy weapons, mortars, tanks and rocket systems are to be removed from the zone by October 10.

Close to 3 million people live in Idlib, around half of them Syrians displaced by the war from other parts of Syria, and the United Nations has warned that an offensive would cause a humanitarian catastrophe.