Beirut (AFP) - A formerly US-backed Syrian rebel group on Saturday rejected a deal between Russia and Turkey to avert a large-scale military assault on rebel-held Idlib province.

The deal makes way for a demilitarised zone between rebel and regime-held areas in and around the northwestern governorate, Syria's last insurgent bastion.

Under the accord reached on September 17 amid mounting fears of a devastating regime offensive, jihadist factions would withdraw their heavy weaponry from the buffer zone.

Pro-Turkey rebels have cautiously accepted the deal, but the Jaysh al-Izza faction on Saturday rejected it, saying the zone to be set up by October 15 would only encompass territory currently under rebel control.

It said the buffer zone should be carved out equally from both rebel-held territory and nearby zones controlled by forces loyal to Syrian President Bashar al-Assad.

"We are against this deal, which eats into liberated (rebel-held) areas and bails out Bashar al-Assad," Jaysh al-Izza head Jamil al-Saleh told AFP.

The group, which according to the Syrian Observatory for Human Rights has around 2,500 fighters, is mainly active in the north of Hama province, bordering Idlib.

It had been close to Ankara but their ties cooled after it refused to join a Turkish-backed alliance, the National Liberation Front.

On September 23, the NLF accepted the demilitarisation deal but said it remained on guard.

A minor Al-Qaeda-linked group, Hurras al-Deen, has also rejected the agreement reached in the Russian resort of Sochi.

The dominant force in the region bordering Turkey, the Hayat Tahrir al-Sham (HTS) alliance led by jihadists of Syria's former Al-Qaeda affiliate, had on Saturday still not responded.