"The rioters took three medical personnel and three guards hostage" but they were released and "at this point the situation is under control," Denisova wrote.

Media reports said the prison was cordoned off by riot police and traffic was restricted in the area.

Nobody was killed but four guards were treated for injuries, police said.

The inmates accused of instigating the riot have been isolated from the other prisoners and an investigation on suspicion of "activities destabilizing the operations of a penitentiary facility" has been opened, they said.

With reporting by UNIAN, Gordon, and Ukrayinska Pravda