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

"The process of withdrawing heavy weapons began this morning and will continue for a number of days," a rebel group commander told Reuters.

The official said the Turkey-backed National Front for Liberation (NFL) rebel alliance will extract its 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.

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 demilitarisation zone.

The 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 3 million people.

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

(Reporting by Suleiman al-Khalidi in Amman; Writing by Lisa Barrington in Beirut; Editing by Ros Russell)