An attack on a checkpoint of the US-backed Syrian Democratic Forces (SDF) in the northern Syrian town on Manbij had left seven fighters dead on Tuesday.
Sharfan Darwish of the Manbij Military Council told the Associated Press (AP) that the attack came shortly after midnight at one of the entrances to Manbij, targeting the fighters who "were carrying out their mission of protecting" the town.
Meanwhile, Britain-based Syrian Observatory for Human Rights, a war monitoring group, said members of an ISIS sleeper cell carried out the attack, and that three other Manbij Military Council fighters were also wounded.
ISIS claimed responsibility for the attack saying its followers targeted a checkpoint on the western edge of Manbij and confiscated the weapons of the US-backed fighters.
Observatory head Rami Abdul Rahman said it was also the bloodiest attack in Manbij since January 16, when 19 people, including four US service personnel, were killed in a suicide bombing claimed by ISIS.
The Observatory said hundreds of SDF members had been killed in attacks believed to have been carried out by ISIS sleeper cells since August.
