The deaths, some of which were confirmed by Reuters journalists at the prison where the men were held in eastern Donetsk province, overshadowed a UN-brokered deal to restart shipping grain from Ukraine and ease a worldwide food crisis.
Forty prisoners were killed and 75 wounded at the prison in the frontline town of Olenivka held by Moscow-backed separatists, Russia's defence ministry said. Reuters TV showed the remains of a cavernous burned-out building filled with metal beds, some with charred bodies lying on them while other bodies were lined up on military stretchers or on the ground outside.
The International Committee of the Red Cross is seeking access to the site and has offered to help evacuate the wounded, ICRC said in a statement on Friday. It said the building was blown up by mercenaries from Russian private military company Wagner Group and was not coordinated with the Russian Defence Ministry.
A spokesman for the Moscow-backed separatists told journalists that Ukraine had attacked the prison after the POWs had begun talking about crimes conducted by the Ukrainian military.