An official with the U.S.-backed force in Syria says gunmen have attacked one of their checkpoints in the northern town on Manbij, killing seven fighters.

Sharfan Darwish, of the Kurdish-led Manbij Military Council, told The Associated Press that the Tuesday morning attack hit a checkpoint at one the entrances of Manbij.

He says it's unclear who was behind the attack and added that the ``martyrs were carrying out their mission of protecting Manbij.''

The Britain-based Syrian Observatory for Human Rights, a war monitor, said sleeper cells of the Islamic State group carried out the attack.

No one immediately claimed responsibility for the attack but in January, IS claimed a suicide attack in Manbij that killed 19 people, including two U.S. service members and two American civilians.