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 sleepers 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.