A bomb attack in the Syrian city of Raqqa killed eight people on Tuesday, including four members 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 the first 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 the hospital told Reuters.
Footage on social media showed rescuers taking mangled and burnt bodies and wounded people to ambulances in the spot where the blast took place, amid flames from a burning vehicle.
There has been a steady increase in attacks on areas controlled by the Syrian Democratic Forces in the swathe of the territory it controls in northeast Syria on the border with Turkey, down the Euphrates River towards the border with Iraq.
There has also been a spate of assassinations of SDF fighters and commanders in recent months, residents say.
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.
The SDF has warned that Daesh militants will intensify their insurgent activities after being defeated in their last enclave in Syria last month.
In a separate attack on Tuesday, a suicide bomber detonated a small truck laden with explosives near a convoy of US-led international 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 noon 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.