Four suicide bombers struck in Syria's Raqqa on Wednesday, a spokesman for the US-backed Syrian Democratic Forces (SDF) which controls the city said in a message to journalists.
A string of bombings have in recent months targeted the northeastern corner of Syria held by the SDF, even after its capture of the last Islamic State enclave in the area.
Islamic State has claimed responsibility for several of those attacks, underscoring its continued threat after its territorial defeat last month at Baghouz on the Euphrates at the Iraqi border.
In January, the group targeted a restaurant in Manbij, a flashpoint city held by a militia allied to the SDF near areas controlled by the Syrian government and Turkey-backed rebels, killing four Americans.
"Four suicide bombers blew themselves up," said Mustafa Bali, head of the SDF's media office, in an online chat when asked about blasts in Raqqa.
The SDF drove Islamic State from Raqqa in 2017 but the fierce military campaign there, including intensive air strikes from a US-led coalition, left much of the city in ruins.
Islamic State fighters are still holding out in a remote area of the Syrian desert and security officials say others have gone underground in Iraqi cities.
Spearheaded by the Kurdish YPG militia, the SDF's hold over northeastern Syria, including much of the border with Turkey, alarms Ankara which regards it as a terrorist group.