RAQQA, Syria (Reuters) - A bomb attack in the Syrian city of Raqqa killed at least eight people on Tuesday, including four fighters 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 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.

Islamic State’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 center 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 toward 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 Islamic State (IS) 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.

Last week Raqqa residents staged a protest against a ban on motorcycles imposed after their widespread use by Islamist militants for hit-and-run attacks on SDF checkpoints and patrols in the city.

Resentment is mounting against Kurdish-led forces among the predominantly Arab population in the vast areas of eastern Syria they rule, with many objecting to compulsory conscription of youths and discrimination on leadership positions, they say.

Many residents of the Arab city that endured heavy aerial strikes by the U.S.-led coalition blamed for hundreds of civilian deaths and destroying much of its infrastructure have been angry at the Kurdish-led administration’s failure to speed up reconstruction or compensate families.

Separately on Tuesday, a suicide bomber detonated a small truck laden with explosives near a convoy of U.S.-led coalition forces in Hasaka province but without causing any casualties, an SDF military source and Kurdish media reported.

There was no immediate comment from the coalition on the attack, which happened just after midday in the town of al-Shadadi, south of the city of Hasaka. The Syrian Kurdish news outlet ANHA said a small flatbed truck was used in the attack.

The Islamic State-affiliated Amaq news agency said an IS suicide bomber had struck a coalition convoy on the main highway between Hasaka and Shadadi but gave no further details.