BEIRUT (Reuters) - A U.S.-backed force fighting to drive Islamic State out of Manbij said on Thursday it was giving the jihadist group 48 hours to pull out of the surrounded northern Syrian city.

A statement from the Manbij Military Council said the Islamic State militants would be allowed to leave the city with light weapons, without a fight.

It said their departure must take place within 48 hours and "this initiative is the only and the final one for the besieged Daesh elements to leave the city," using an Arabic acronym for the jihadist group.

The council is allied to the U.S.-backed Kurdish and Arab alliance known as the Syria Democratic Forces (SDF), which has been fighting Islamic State in northern Syria with the support of air strikes from a U.S.-led coalition.

The SDF fighters, who quickly took control of the territory surrounding Manbij last month, have in recent weeks taken western parts of the city in slower advances.

Activists say dozens of civilians have been killed this week in air strikes in the city and to the north, and the opposition Syrian National Coalition called for a suspension of the air strikes while the incidents are investigated.

(Reporting by Dominic Evans; Editing by Andrew Heavens)