The photo has been used for illustrative purposes. File/Reuters

A bomb attack in the Syrian city of Raqqa killed at least eight people on Tuesday, including four fighters of the US-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 an initial blast, which was followed by a bigger explosion that caused the casualties.

"I heard the sound of a big explosion and people were rushing and I saw people blown up with body parts," an injured witness who was rushed to hospital told Reuters.

Daesh's Amaq news agency said the group's militants entrapped Kurdish security forces by detonating a sound explosive device on a main street and after people congregated they detonated a car that was parked near by.

Footage on social media showed rescuers taking mangled and burnt bodies and wounded people to ambulances in the city centre near the headquarters of the security forces where the blast took place, amid flames from a burning vehicle.

There has been an steady increase in attacks on areas controlled by the Syrian Democratic Forces in the swathe of territory it controls in northeast Syria on the border with Turkey, down the Euphrates River towards the border with Iraq.

Residents have also reported a spate of assassinations of SDF fighters and commanders in recent months.

The SDF has warned that Daesh militants would intensify their insurgent activities after being defeated in their last remaining enclave in Syria last month. The SDF ended IS control of Raqqa in October 2017 after months of fighting.

Reuters