Russian special forces killed several prisoners linked to ISIS and freed two guards taken hostage at a pre-trial detention centre on Sunday.
Six prisoners at Detention Centre No 1, in the southern Rostov region, armed themselves with a penknife, a rubber baton and a fire axe before knocking out the bars of the window in their cell and entering a guard room, where they took two officers hostage.
Some of the prisoners include men with links to ISIS, which claimed responsibility for a deadly attack on a Moscow concert hall in March.
"The criminals were eliminated," Russia's Federal Penitentiary Service said in a statement. It said a "special operation" has been launched to free the hostages.
"The employees who were being held hostage were released. They are uninjured," the prison service said.
A number of local news outlets, including RT, reported that at least some of the prisoners had been killed.
RT reported that three of the hostage-takers had already been sentenced on terrorism charges, including disseminating extremist information and plotting to blow up a Russian courthouse.
Gunfire could be heard in footage published on Russian Telegram channels. Before special forces stormed the detention centre, one of the hostage takers was shown on the 112 Telegram channel brandishing a knife beside one of the guards.
The prisoner was wearing a headband bearing the ISIS flag.
The prisoners demanded a car and free passage, the Interfax news agency reported.
The attack at a Moscow concert hall killed 145 people and injured dozens. ISIS has claimed responsibility for numerous attacks on Russian interests in the past, including the 2015 bombing of Metrojet Flight 9268.
The attack killed 224 as they travelled to St Petersburg from the Egyptian resort of Sharm El Sheikh.
