A group of prisoners seized hostages in a jail in Russia's Volgograd region on Friday and at least one member of the prison staff was killed, state media said.
"During a session of a disciplinary commission, convicts took staff of the penal institution as hostages," the prison service said in a statement.
"Measures are currently being taken to free the hostages. There are casualties."
The prison in the town of Surovikino is designated as a "harsh regime" penal colony with capacity to hold up to 1,241 male prisoners, according to the prison service website.