A bomb attack in the Syrian city of Raqqa killed eight people on Tuesday, including four members of the U.S.-backed Syrian Democratic Forces, a security source in northern Syria said.

The blast also killed four civilians and wounded three members of the Kurdish-led internal security forces known as the Asayish, the source said.

Witnesses said people including SDF members gathered at the scene after the first blast, which was followed by a bigger explosion that caused the casualties.