Russian inmates on Friday took staff hostage at a prison colony in the southern Volgograd region during attacks that officials said left four workers hospitalised.
The siege comes two months after Islamic State group-tied prisoners took guards hostage in another facility and as ethnic tensions run high following the IS-claimed Moscow concert hall attack.
"Convicts took hostage employees of the (IK-19) correctional institution. Measures are currently being taken to free the hostages. There are casualties," Russia's federal penitentiary service said in a statement.
Four prison staff were hospitalised due to the attack, Volgograd governor Andrey Bocharov said.
"An operational headquarters has been set up to coordinate the work on releasing the hostages. Law enforcement and security agencies are carrying out operational activities. There is no threat to the civilian population," he added.
Russian President Vladimir Putin said the "situation" would be discussed at a regular meeting of the country's Security Council on Friday.
Videos published on Russian social media appeared to show prison guards covered in blood after being taken hostage.
The clips also showed the hostage-takers holding flags affiliated with IS.
AFP could not verify those images.
Russia's investigative committee said several prisoners were involved.
Governor Bocharov did not comment on the identity of the hostage takers but alluded to reports they were not Russian citizens.
"Everyone on our territory is obliged to respect and comply with the laws of Russia. We will not allow anyone to try to incite ethnic discord," he said in a statement published by the regional administration.
The IK-19 prison colony is located in the town of Surovikino, around 850 kilometres (530 miles) south of Moscow.
The incident happened during a meeting of the prison's disciplinary commission, the Federal Penitentiary Service added.
In June, prisoners aligned with IS staged a similar siege at a jail in the southern Rostov region.
Russian special forces then managed to kill the hostage-takers and free the guards after a stand-off.
Tensions over migration are running high in Russia following the attack on a Moscow concert hall in March that killed 145 people. It is the most deadly terror attack in Russia for two decades.
A Central Asian branch of IS claimed responsibility for the attack and the four suspected gunmen, now in pre-trial detention, are citizens of Tajikistan.
Millions of people from Central Asia, which was part of the Soviet Union, live in Russia, many working low-skilled jobs to send money back to families.
IS has repeatedly pledged to target Russia over its support of Syrian leader Bashar al-Assad, who has waged a military campaign to quash the group in the Middle East.
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