A car bomb and explosive ordinance struck a crowded street in Raqqa, Britain-based Syrian Observatory for Human Rights says.
A double bomb attack on Tuesday in the northern Syrian city of Raqqa, a former bastion of the Daesh terror group, killed at least 15 people, mostly civilians, a war monitor said.
A car bomb and explosive ordinance struck a crowded street, killing at least nine civilians and four US-backed militants, the Britain-based Syrian Observatory for Human Rights said.
A Raqqa resident said he saw black smoke rising above the site of the attack as ambulance sirens rang out.
He said he saw "blotches of blood on the ground".
"There is panic and fear in the area after the bombings," he added.
There was no immediate claim of responsibility for the attack near a military outpost of the US-backed PYD/YPG-dominated SDF militia who have been battling Daesh in Syria.
The YPG is the Syrian offshoot of the PKK terrorist organisation. In its 30-year terror campaign against the Turkish state more than 40,000 people, including women and children, have been killed.
Turkey, the US and the EU recognise the PKK as a terrorist organisation.
The city of Raqqa was captured by the SDF in 2017 after a massive operation against Daesh.
The SDF said it had defeated the terror group's so-called "caliphate" last month, after a months-long offensive in Syria's east.
Daesh fighters, however, retain a presence in Syria's vast Badia desert and various other hideouts and continue to claim deadly attacks in SDF-occupied territory.
Earlier on Tuesday, a car bomb targeted a patrol of the US-led coalition in the northeastern town of Shadadi, the war monitor said.
But it failed to hit its target, leading only to the death of the suicide bomber, it said.
Last month, Daesh fighters killed seven US-backed fighters in an attack on the northern city of Manbij.
The US coalition has warned that a new phase has begun in anti-Daesh operations, and appealed for sustained coalition assistance to help hunt down sleeper cells.
Daesh seized a large swathe of Syria and neighbouring Iraq in 2014, declaring a "caliphate" in the lands it controlled.
After various offensives saw the Daesh group lose their main cities in 2017, the SDF on March 23 said they had vanquished its last patch of territory in the eastern Syrian village of Baghouz near the Iraqi border.
At the height of their rule, the terror organisation ruled millions of people in a proto-state the size of the United Kingdom.
They collected taxes, minted their own coins, and meted out brutal punishment on anyone who disobeyed or opposed them.
Saturday's deaths are the latest in Syria's devastating eight-year war, which has killed hundreds of thousands of people since starting in 2011 with the brutal repression of anti-regime protests.