The hostages at the pre-trial detention centre in Rostov-on-Don were reportedly uninjured, said Russia's Federal Penitentiary Service.

Media reports from the scene said that sounds of gunfire could be heard, while multiple ambulances were seen arriving at the prison in footage on social media.

The news agency's report said that the hostages had been "liquidated", with other media reports suggesting that at least some prisoners were killed.

State-funded news channel RT reported that three of the hostage-takers had been sentenced on terrorism charges, including disseminating extremist information and plotting to blow up a Russian courthouse.

Purported visuals of the incident that surfaced on social media appeared to show at least two of the hostage-takers wearing a headband that resembled the Islamic State's flag. Other images showed prisoners wielding knives.

Earlier, six hostage takers were in the central courtyard of the Rostov region's Detention Centre No.1, state news agency TASS reported citing sources in law enforcement. The hostage takers were reportedly armed with penknife, a rubber baton and a fire ax.

Among the prisoners are those who also have been accused of having links to the Islamic State (IS) group, the report said.

In the recent years, IS have carried several attacks on Russian soil, including the most recent one in March where a shooter opened fire at crowd at a concert hall in suburban Moscow. The gunman had killed 145 people.