Islamic State “sleeper cells … infiltrated from the surrounding neighbourhoods and clashed with the internal Security Forces,” the U.S.-backed group added.

The SDF, which is spearheaded by the Kurdish YPG militia, said its forces thwarted the escape attempt by prisoners that coincided with a car bomb that was detonated near the prison by militants, spokesman Farhad Shami said in a tweet.

There were unconfirmed reports that several inmates had been killed in the mutiny, the latest of several recent attempts to flee from SDF prisons, according to two residents.

Arab tribal figures in touch with residents in the area said U.S. coalition planes were seen flying overhead in the vicinity of the prison following the incident.

It was not clear how many inmates were in the prison, one of several where the SDF has kept thousands of detainees, many of whose relatives say are young children and others arrested on flimsy charges or for disobeying the SDF’s policy of forcible conscription. (Reporting by Suleiman AL-Khalidi Writing by Ahmad Elhamy Editing by Chris Reese and Jonathan Oatis)