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. 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 ISIS enclave in the area. ISIS 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. The SDF drove ISIS 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. ISIS fighters are still holding out in a remote area of the Syrian desert and security officials say others have gone underground in Iraqi cities.