AMMAN (Reuters) - U.S.-backed forces in control of Syria’s Raqqa city imposed a three-day curfew on Sunday and declared a state of emergency saying Islamic State militants had infiltrated the city and planned a bombing campaign.

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Witnesses say internal security forces affiliated to Syrian Democratic Forces (SDF), who had announced late last night the surprise curfew from dawn in the former Islamic State stronghold until Tuesday, set up checkpoints across the devastated city.

A statement said the state of emergency in the city were “protective measures”.

“Our intelligence apparatus has received information of the entry of terrorist groups tied to Daesh (acronym of IS) that are about to wage attacks to undermine secuity in the city,” the statement read.

The curfew prevents anyone from leaving or entering the city that was retaken by U.S.-backed militias last October, after a four-month battle that left thousands of civilians dead and most of its neighborhoods flattened in devastating U.S. led aerial strikes.

Islamic State declared a caliphate in 2014 and at the height of its power ruled over millions of people in parts of Syria and Iraq. The militant group had declared Raqqa the capital of its self-proclaimed state.

Two residents said the city has seen in recent weeks growing tensions between the majority Arab residents and SDF forces dominated by Kurdish personnel that spilled into sporadic protests last month. The protests, quelled by force, called for the SDF’s eviction from the city.

They said many of its over 100,000 residents are angered by a policy of obligatory military conscription of youths and resentful of perceived discriminatory practices against the majority Arab population by senior Kurdish officials who effectively run the city.

The SDF, an alliance of Kurdish and Arab fighters, denies these accusations and says its local councils are run by the city’s residents and says it seeks to redress decades of ethnic discrimination against Kurds by the Syria’s ruling Baath party

Another resident in touch with local officials said the security operation aimed at arresting the leader of a disaffected Arab rebel group Thwar al Raqqa, affiliated with the SDF. A previous attempt last month to arrest him provoked violent demonstrations by his supporters.