Turkey on Thursday said its armed forces were engaged in "clean-up" operations to clear remaining IS militants from the flashpoint Syrian town of Al-Bab after a weeks-long campaign.

The Turkish army, backing Syria rebels, have since December been engaged in fierce fighting to oust the jihadists from the town but Ankara now says Al-Bab is largely under its control.

"Al-Bab is now completely surrounded," Defence Minister Fikri Isik told Turkish media in Brussels where he was attending a NATO meeting.

"There is a serious clean-up going on inside to clear Daesh (IS) completely," he added. "Once this clean-up is completed, we expected life in Al-Bab to return to normal."

Turkey's offensive has been matched by a separate operation by forces of Syrian President Bashar al-Assad on the town from the south.

The Britain-based Syrian Observatory for Human Rights however said Turkish forces had made little progress since entering the town from the west.

It accused Turkey of killing 24 civilians in air strikes. But Turkey's army said it had killed 15 "terrorists" in air strikes, artillery fire and clashes.

Isik said the Turkish forces subsequently wanted to move on the town of Manbij but wanted the Kurdish militia fighters who ousted IS there last year to leave first.

Isik reaffirmed Turkey was also looking into the possibility of a joint operation with the United States and other powers to take the jihadist bastion of Raqa from IS.

But he again insisted that the Kurdish militia -- enemies of Turkey but allies of the US in the fight against IS -- must not be involved in the operation.

Isik announced that US Chairman of the Joint Chiefs of Staff Joseph Dunford would be visiting Ankara on Friday for talks about the issue.

"We will consider whether to do the Raqa operation together," Isik said. "I don't think the US has taken a definite decision on this," he added.