BEIRUT-- Airstrikes on villages held by the Islamic State of Iraq and Syria (ISIS) in northern Syria killed at least 56 civilians on Tuesday as intense fighting was underway between the militants and U.S-backed fighters, Syrian opposition activists and the extremist group said.

Residents in the area blamed the U.S.-led coalition for the strikes that targeted two villages, Tokhar and Hoshariyeh, which are controlled by ISIS, activists said. The villages are near the ISIS stronghold of Manbij, a town that members of the predominantly Kurdish U.S.-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.

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The ISIS-linked Aamaq news agency claimed 160 civilians - mostly women and children - were killed in Tokhar alone, in a series of purportedly American airstrikes around dawn Tuesday.

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 grave, 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 AP reporting of the events depicted. Tuesday's casualties come on the heels of similar airstrikes on the ISIS-held town of Manbij on Monday, when at least 15 civilians were reportedly killed.

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

The Manbij area has seen intense battles between ISIS extremists and the Kurdish-led fighters who have been advancing under the cover of intense airstrikes by the U.S.-led coalition.

If Manbij is captured by the U.S.-backed fighters, it will be the biggest strategic defeat for ISIS in Syria since July 2015, when the extremist group lost the border town of Tal Abyad. In neighboring Iraq, meanwhile, ISIS has been beaten back on several fronts, with Iraqi forces, aided by U.S.-led coalition airstrikes, having retaken the cities of Ramadi and Fallujah in western Anbar province.