Unverified Russian video footage of the aftermath shows a tangle of wrecked bunk beds and badly charred bodies.

Exactly what happened at the prison camp in Olenivka, which is controlled by the Russian-backed self-proclaimed Donetsk People's Republic (DNR), remains unclear.

Ukraine says the site was targeted by Russia in an effort to destroy evidence of torture and killing. President Volodymyr Zelensky described the incident as a "deliberate Russian war crime."

For its part, Russia said the camp was hit by Ukrainian precision rockets.

Those detained at the prison are said to have included members of the Azov battalion, who were captured defending the southern city of Mariupol in May and whom Russia has sought to depict as neo-Nazis and war criminals.

On Friday, after the prison attack, the Russian embassy in the UK tweeted that Azov "militants deserve execution, but death not by firing squad but by hanging, because they're not real soldiers. They deserve a humiliating death".

Daniil Bezsonov, a spokesman for the DNR, said the strike had been a "direct hit on a barracks holding prisoners".

Russia's defence ministry said the strike had been carried out with US-made Himars artillery and it accused Ukraine of a "deliberately perpetrated" provocation. The ministry produced fragments of what it said were rockets fired by the Himars system.

But Ukraine denied that any rocket or artillery strikes had been made.

An adviser to President Zelensky said the scene looked like arson, and that a missile strike would have scattered the bodies.

Ukraine's general staff of the armed forces called on the UN and the Red Cross to investigate the deaths, claiming that Russia had targeted the camp in order to cover up its treatment of POWs.

Writing on social media, it said they should immediately respond as the two organisations had given guarantees that the prisoners of war would be kept safe there.

The Red Cross said it was seeking access to the site and had offered to help evacuate the wounded.

"Our priority right now is making sure that the wounded receive life-saving treatment and that the bodies of those who lost their lives are dealt with in a dignified manner," it said in a statement.

Ukraine's new Prosecutor-General Andriy Kostin earlier said he had opened a war crimes investigation into the blast.