The US-backed Syrian Democratic Forces said four Internal Security Forces fighters and two SDF members were killed during an ISIS attack on one of their centres on Monday morning.
One ISIS fighter was killed and another who was wearing a suicide belt was arrested while fleeing, said SDF spokesman Farhad Shami.
“Others managed to flee to the adjoining neighbourhoods,” he said.
The SDF said it imposed a curfew across Al Raqqa, where the incident took place, in anticipation of further ISIS attacks and until it finds the perpetrators.
“Following the attack, our forces imposed a complete curfew, launched a large-scale sweep operation in the city to foil the dangerous and continuous terrorist attack.”
Syria's northern city of Al Raqqa was the last ISIS stronghold until 2017, when the group was territorially defeated by the SDF.
On Monday, the UK-based Syrian Observatory for Human Rights said it had documented 16 ISIS attacks against the SDF since the beginning of the month.
“These attacks left 11 people, including nine SDF members, dead and nine civilians and combatants injured,” the observatory said. It is not known whether the tally included Monday's ISIS attack.
The US trains the SDF and provides them with intelligence in their fight against ISIS
Last month, the SDF temporary halted anti-ISIS operations while anticipating a ground offensive by Turkey following an announcement by President Recep Tayyip Erdogan that his forces will be targeting Kurdish militants.
Turkey said Mr Erdogan's decision was in response to an attack on a crowded tourist spot in Istanbul that killed six people in November. Ankara blamed armed Kurdish militants for the attack, but the SDF denied responsibility for it and called for an international investigation to be conducted into the incident.