The incident follows a similar attack in a prison in Rostov-on-Don in June. The hostage-takers claimed to be part of the Islamic State terrorist group.
ADVERTISEMENTRussian special forces have killed four prisoners at a penal colony in the southwest of the country who were reported to have taken hostages and killed a member of staff.
The National Guard of Russia, the country's gendarmerie, announced the operation on Friday after news emerged of the hostage situation. All the individuals taken have now been freed, it said.
The incident occurred at penal colony 19 in the town of Surovikino, 130 kilometres west of Volgograd. Authorities have not yet specified the exact number of attackers, hostages and victims, and the information circulating on various Telegram channels differs widely.
An explicit, unverified video circulating on several Russian social media channels shows at least three assailants brandishing knives and standing over what appear to be injured or dead prison guards, lying in a pool of blood.
One of the hostage-takers states in the clip that the group is affiliated with the Islamic State (or Daesh). According to several local channels, the attackers demanded a helicopter and $2 million (€1.8 million).
Russian media reported that the country's security services were preparing to storm the building.
Russian security under strain amid Ukraine war?
Russia has recently experienced a series of Islamist terrorist attacks, according to various media, raising the question of whether its vast security agencies have been stretched thin by Moscow's full-scale invasion of Ukraine and its subsequent internal crackdown on anti-war dissent.
An Afghan branch of the Islamic State claimed responsibility for a mass shooting at a concert hall near Moscow in March — the deadliest terrorist attack in the country for years — which left at least 145 people dead and 551 injured.
In June, a synagogue was attacked in the city of Derbent, while an Orthodox church was targeted by gunmen further south, in Makhatchkala, Dagestan (North Caucasus). At least 19 people were killed, including an Orthodox priest and 15 policemen, according to the authorities, who denounced these as "terrorist acts".
Yet more questions about Russia's prison security measures were raised when security forces had to storm a detention centre in the south of the country, also in June, killing six inmates who had taken two staff members hostage. The prisoners included men who were also accused of being linked to the Islamic State.