Russian inmates have taken staff hostage at a prison colony in the southern Volgograd region in the second such event since June, officials said.
At least one member of the prison staff was killed in the incident on Friday, state media reported.
“During a session of a disciplinary commission, convicts took staff of the penal institution as hostages,” Russia’s federal penitentiary service said in a statement.
“Measures are currently being taken to free the hostages. There are casualties,” the statement added.
Russia’s investigative committee said “several prisoners” were involved.
The IK-19 prison colony is located in the town of Surovikino, about 850km (530 miles) south of the capital, Moscow.
Videos published on Russian social media appeared to show about four prison guards taken hostage, some lying in pools of blood.
A video posted by the news channel Mash showed at least two attackers. One of them shouted that they were “mujahideen” of the ISIL (ISIS) group and had seized control of the prison. The video suggested the death toll was higher.
The videos could not be independently verified.
Russian President Vladimir Putin said in televised remarks that he had been briefed by the head of the prison service.
The prison is designated as a “harsh regime” penal colony with the capacity to hold up to 1,241 male prisoners.
News sites with security connections published the names of up to four alleged attackers, identifying them as citizens of Uzbekistan and Tajikistan. There was no official confirmation.
The attacks come after the Islamic State in Khorasan Province, an ISIL affiliate, claimed responsibility for an attack on a concert hall in Moscow that killed 145 last March, the deadliest such strike in Russia for two decades.
The group remains one of the most active affiliates of ISIL and takes its title from an ancient caliphate in the region that once encompassed areas of Afghanistan, Iran, Pakistan and Turkmenistan.
It emerged from eastern Afghanistan in late 2014 and was made up of breakaway fighters of the Pakistan Taliban and local fighters who pledged allegiance to the late ISIL leader, Abu Bakr al-Baghdadi.
In June, a bloody ISIL-linked prison uprising took place in the southern region of Rostov, where special forces shot dead six inmates who had taken hostages.
Later that month, at least 20 people were killed in shooting attacks in two cities in Dagestan, a mainly Muslim region of southern Russia.
ISIL has repeatedly pledged to target Russia over its support of Syrian President Bashar al-Assad, who has waged a military campaign to quash the group in the Middle East.
