BEIRUT — Airstrikes on Islamic State-held villages 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 – as the group is commonly called – 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 death toll from the airstrikes, which coincided with a wide ground offensive by the extremists against SDF fighters, ranged between 56 and 200. If it is confirmed that 200 people were killed, it would be the deadliest strike by the U.S.-led coalition since it began its military campaign against ISIS in Syria almost two years ago.

Conflicting numbers in the aftermath of attacks are not uncommon in Syria. There were also conflicting reports on where the civilians were killed, with some groups reporting that a school housing refugees was hit and others saying that people were struck as they fled the violence.

An international human rights group said the U.S.-led coalition, which has been carrying out airstrikes against ISIS in Syria since September 2014, must increase its efforts to prevent civilian deaths and investigate possible violations of international law.

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 90 people were killed.