U.S.-backed fighters in northern Syria have renewed an offer to Islamic State militants in the besieged northern town of Manbij, saying the extremists can leave it and would not be attacked.

The Syria Democratic Forces says the offer is meant to protect civilians in the town.

The predominantly Kurdish force has been on the offensive in Manbij, backed by U.S.-led coalition airstrikes.

Monday's offer by the SDF-linked Manbij Military Council comes days after the extremists ignored an earlier, 48-hour offer to leave the town just with their "individual weapons."

It says that if IS allows all civilians to leave, SDF would in return allow wounded IS militants safe passage to other areas nearby under their control.

The council urged IS to send a delegation from Manbij to discuss the matter.