A final assault on Islamic State in Syria's Raqqa should begin on Sunday night, a field commander for the Syrian Democratic Forces (SDF), backed by a U.S.-led coalition, said on Sunday.

The assault would focus on the area around the stadium and attempt to surround it, the field commander, Ardal Raqqa, told Reuters in the city.

The spokeswoman for the SDF campaign in Raqqa, Jihan Sheikh Ahmad, said in a statement on a website for the campaign that it would announce the liberation of Raqqa "in the coming few days".

Turkish military vehicles entered Syria's Idlib province on Sunday under escort from the Tahrir al-Sham jihadist alliance, a resident of the Syria-Turkey border area and a local rebel said.

They both said they saw several vehicles carrying Turkish forces entering Idlib at the Atmah crossing before Tahrir al-Sham fighters escorted them along a road.

On Saturday, Turkish President Tayyip Erdogan said Syrian rebels backed by Turkish forces would launch an operation in that area, which is mostly controlled by Tahrir al-Sham.

On Sunday, a war monitor reported clashes between Turkish forces and the jihadists.

The incursion follows a deal between Turkey and Syrian President Bashar al-Assad's allies Russia and Iran to impose a "de-escalation" zone in Idlib and surrounding areas to reduce warfare there, an agreement that did not include Tahrir al-Sham.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said the clashes took place near the village of Kafr Lusin in Idlib.

Tahrir al-Sham is spearheaded by the former Nusra Front, which was al Qaeda's Syrian branch until last year, when it changed its name and broke formal allegiance to the global movement founded by Osama bin Laden.