Kyiv has accused Russia of the deaths of dozens of Ukrainian soldiers in an attack on a facility where they were being detained in the country's pro-Russian-forces controlled area.

Ukrainian President Volodymyr Zelenskyy said on Friday that more than 50 people were killed in the attack on Olenivka prison in the eastern region of Donetsk. He said, "It was a deliberate Russian war crime, a deliberate mass murder of Ukrainian prisoners of war."

Ukrainian prosecutors say they have begun an investigation to pursue Russia's responsibility for the incident.

Russia's defense ministry also reported the attack on Friday. Video footage from Russia's state-run news agency shows a shattered facility with destroyed ceilings and rows of charred bunk beds.

The ministry said the strike was delivered using a US-made HIMARS rocket system, and called it a provocation from the Ukrainian side.

The General Staff of the Armed Forces of Ukraine has categorically denied Russia's claims. It says, "The committed explosion is a cynical terrorist act of Russian Federation, a military provocation and a typical false flag operation, the purpose of which is to cover up war crimes."

EU foreign policy chief Josep Borrell issued a statement over the prison attack. He said, "The European Union condemns in the strongest possible terms the atrocities committed by the Russian forces and their proxies."