Local security forces in Syria's northern city of Raqqa announced a two-day curfew starting Sunday in response to intelligence on potential attacks by the Islamic State group.
Two mass graves containing dozens of bodies of civilians and Syrian troops killed by Islamic State jihadists have been found in the west of Raqqa province, state news agency SANA reported Friday.
The separate assaults across the Iraqi-Syrian frontier aim to deal a final blow to the Islamic State "caliphate" that has crumbled this year in Syria and Iraq.
Raqqa will be part of a decentralised federal Syria now the city has been freed from Islamic State, the US-backed militias that captured it said on Friday, tying its political future to Kurdish-led autonomy plans for northern Syria.
The Syrian Democratic Forces said the handover would not be immediate, with SDF fighters still combing the city of Raqqa.
Days ago in Syria's Raqqa, die-hard jihadists made their final stand in the city's national hospital. On Wednesday, it stood eerily silent, its entrance inhabited only by swarming flies and two rotting corpses.
US-backed forces took full control of Raqqa from the Islamic State group on Tuesday, defeating the last jihadist holdouts in the de facto Syrian capital of their now-shattered "caliphate".
Raqqa was once the de facto Syrian capital of the jihadists' self-styled "caliphate" in Syria and Iraq, but IS now holds just 10% of the city.
Islamic State is on the verge of defeat in Syria's Raqqa and the city may finally be cleared of the jihadists on Saturday or Sunday, the Syrian Kurdish YPG militia said on Saturday.
Among the ruins of a street in Syria's Raqqa, an anti-jihadist fighter's call echoes from a loudspeaker: "Civilians, we are waiting for you to leave to safety."