Prisoners seize hostages in Russia's Volgograd region

At least four uniformed prison officials could be seen lying or sitting in pools of blood in the video.

"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 is designated as a "harsh regime" penal colony with capacity to hold up to 1241 male prisoners, according to the prison service website.