BEIRUT (Reuters) - Syrian Turkey-backed rebel groups began to withdraw heavy weaponry from a demilitarized zone agreed by Turkey and Russia in northwest Syria on Saturday, rebels said.

The Turkey-backed National Front for Liberation (NFL) rebel alliance said in a statement the process of withdrawing heavy weapons had begun, but the fighters would remain in their positions within the demilitarized zone.

Under the deal agreed last month between Turkey and Syrian President Bashar al-Assad’s key ally Moscow, “radical” rebels will be required to withdraw by the middle of this month from the zone, and heavy weaponry must be withdrawn by Oct. 10.

The NFL statement said the Syrian rebels would remain within the demilitarized zone to assist Turkish troops monitoring and patrolling the area.

Two Turkey-backed Syrian rebel officials told Reuters that the removal of heavy weapons began on Saturday and would last for a number of days.

A rebel group commander told Reuters the NFL will extract heavy weaponry - such as rocket launchers and artillery vehicles - and bring it 20 km (12 miles) from the contact line between insurgents in Syria’s rebel-held Idlib province and government forces.

“Light and medium weapons and heavy machine guns up to 57 mm will remain in place,” the official said.

The Turkey-Russia agreement halted a threatened Syrian government offensive. The United Nations had warned such an attack would create a humanitarian catastrophe in the Idlib region, home to about three million people.

The main jihadist group in the Idlib area, Tahrir al-Sham, has yet to say whether it will comply with the agreement.