US-backed SDF forces give IS 48 hours to leave Syria's Manbij

US-backed fighters on Thursday gave the Islamic State (IS) group 48 hours to leave the battleground Syrian town of Manbij, after a weeks-long offensive that has seen intensive air strikes by the US-led coalition accused of killing scores of civilians. 

The 48-hour ultimatum was issued by the Arab-Kurdish Syrian Democratic Forces (SDF), who are fighting IS with coalition air support.

The SDF has been waging a major campaign since late May to oust IS from Manbij, in Aleppo province, with US-led air support.

The deadline follows a major backlash after reports that at least 56 civilians, including several children, were killed in air strikes by the coalition near Manbij on Tuesday, an attack which followed what one monitor said is the deadliest week for civilians since the US-led coalition entered the war.

"In order to protect civilian lives and property and to protect the town from destruction, we announce that we accept the initiative under which besieged IS members would leave with their individual light weapons," said the Manbij Military Council, part of the SDF.

"This initiative is the last remaining chance for besieged members of Daesh (IS) to leave the town."

An SDF commander said the initiative was first floated last week by tribal leaders in Manbij, which is a key IS bastion.

"But we took this decision now after IS used residents as human shields, after the media pressure on us, and to protect whatever civilians are left in the town," he said on condition of anonymity.

The statement also urged civilians to try to leave Manbij or distance themselves from areas where clashes are taking place.

The Syrian Observatory for Human Rights, a Britain-based monitor, said the civilians killed on Tuesday were villagers fleeing fighting in Tukhar, 14 kilometres from Manbij.

On Thursday afternoon, the Observatory reported quiet in Manbij, though it was unclear if the calm was temporary or in response to the SDF's ultimatum.

Earlier in the day, the monitor had reported additional US-led air strikes and it said the SDF had advanced inside the town overnight.