Air strikes on Islamic State-held villages in northern Syria have killed at least 56 civilians, amid intense fighting between the militants and US-backed forces.

Residents in the area blamed the US-led coalition for the strikes that targeted two villages, Tokhar and Hoshariyeh, which are controlled by IS, activists said.

The villages are near the IS stronghold of Manbij, a town that members of the predominantly Kurdish US-backed Syria Democratic Forces have been trying to capture in a weeks-long offensive.

The Britain-based Syrian Observatory for Human Rights said at least 56 civilians, including 11 children, were killed in the strikes on the villages, which also wounded dozens.

Another activist group, the Local Coordination Committees, said dozens of civilians, mostly families, were killed.

The IS-linked Aamaq news agency claimed 160 civilians - mostly women and children - were killed in Tokhar alone, in a series of purportedly American air strikes at dawn.

The reports and the disparate casualty tolls could not be independently confirmed because the area is inaccessible to independent media. There was no immediate comment from Washington.

Postings on a Facebook page show images of people, including children, as they were being put in collective graves, purportedly in the village of Tokhar. One photograph shows a man carrying the lifeless body of a child covered with dust while another shows a child, partly covered by a blanket, lying in a grave.

The photographs appeared genuine and corresponded to other reporting of the events depicted. Tuesday's casualties come on the heels of similar air strikes on the IS-held town of Manbij on Monday, when at least 15 civilians were reportedly killed.

The US Central Command said the coalition conducted 18 strikes on Monday and destroyed 13 IS fighting positions, seven IS vehicles and two car bombs near Manbij.

The Manbij area has seen intense battles between IS extremists and the Kurdish-led fighters who have been advancing under the cover of intense air strikes by the US-led coalition.

If Manbij is captured by the US-backed fighters, it will be the biggest strategic defeat for IS in Syria since July 2015, when the extremist group lost the border town of Tal Abyad.

In neighbouring Iraq, meanwhile, IS has been beaten back on several fronts, with Iraqi forces, aided by US-led coalition air strikes, having retaken the cities of Ramadi and Fallujah in the western Anbar province.