Russia and Ukraine have blamed each other for a rocket attack that killed dozens of Ukrainian prisoners of war in the occupied part of Donetsk region.

Ukraine says the prison was targeted by Russia in an effort to destroy evidence of torture and killing.

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

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

The BBC cannot independently verify the footage, but analysis suggests the building was hit with an incendiary device, not regular artillery.

An adviser to President Volodymyr Zelensky said the scene looked like the result of arson after a mass murder.

A missile strike would have had the bodies scattered, said the adviser, Oleksiy Arestovych.

Those detained are said to have included members of the Azov battalion, who were the last defenders of Mariupol and whom Russia has sought to depict as neo-Nazis and war criminals.

“The Russian occupants pursued their criminal goals, thus, to accuse Ukraine of committing ‘war crimes’, and to hide the torture of prisoners and shootings committed there by the orders of the occupation administration and the command of the Armed Forces of the Russian Federation,” Ukraine’s General Staff said.

"The Russian enemy continues his propaganda methods of waging information warfare in order to accuse the Armed Forces of Ukraine of shelling civilian infrastructure and the population, hiding his own insidious actions."

Ukraine's new Prosecutor-General Andriy Kostin said he had opened a war crimes investigation into the blast. His office added that it believed about 40 people were killed in the strike and 130 were injured.

Daniil Bezsonov, a spokesperson for the Russian-backed separatist Donetsk People's Republic, said the strike had been a "direct hit on a barracks holding prisoners" and the number killed might increase.

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's spokesperson, Lt Gen Igor Konashenkov, said eight prison staff were also injured in the blast.

But Mykhailo Podolyak, another adviser to President Zelensky, said the Russian allegations were "a classic, cynical and elaborate false flag operation" designed to discredit Ukrainian authorities.

"We demand a reaction from the UN and international organisations," he added.

And Ukraine's Foreign Minister Dmytro Kuleba accused Russia of perpetrating "another petrifying war crime".

Andriy Biletskyi, a founder of the nationalist Azov Battalion, said a number of the unit's soldiers were among those killed.

It has been established that a number of Ukrainian troops who surrendered to Russian forces after a protracted siege at the Azovstal steel plant in Mariupol were taken to Olenivka.

The troops had held out against a relentless Russian assault on the sprawling site for weeks and surrendered in May after using the plant's maze of tunnels to shelter from artillery and air strikes.

Ukrainian officials have said that prisoners-of-war at Olenivka were tortured. (BBC)

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