A Kurdish-led force arrested 63 suspected militants in the Syrian city of Raqa Thursday during an operation against jihadist sleeper cells, it said in a statement.
The US-backed Syrian Democratic Forces said those detained belonged to "terrorist cells directly responsible for spreading terror and chaos" in the city.
At least 48 suspected members of the Islamic State group were among those arrested, according to Rami Abdel Rahman, the head of the Britain-based Syrian Observatory for Human Rights.
He said that members of rival opposition groups are also among those detained by Kurdish security forces in Raqa, once the de facto capital of IS's self-proclaimed "caliphate".
The arrests are part of a sweeping crackdown on alleged IS sleeper cells in territory controlled by the SDF, the Observatory said.
The SDF has whittled down IS territory in Syria to a tiny sliver of land near the border with Iraq.
Jihadists have responded by ramping up bomb attacks and assassinations targeting SDF forces elsewhere.
The city of Raqa, which was taken by the SDF in 2017 after a massive operation by the US-led coalition, has been hit by a spate of bombings in recent weeks.
An explosion on Monday wounded Kurdish security forces in the city, the Observatory said. Last month, an IS suicide bomber attacked a centre for Kurdish forces, killing four civilians and a Kurdish fighter.
The Observatory says that suspected IS sleeper cells have allegedly assassinated at least 50 civilians and 135 SDF fighters in Kurdish-held territory, including eastern Deir Ezzor, Hasakeh's countryside, Raqa and Manbij since August.