Ukrainian President Volodymyr Zelensky said on Friday that an attack on a prison in separatist-held eastern Ukraine that left at least 50 inmates dead was a “deliberate war crime by the Russians”. Russia, meanwhile, blamed Ukraine for the attack. Olenivka prison near Donetsk has been used to house many of the Ukrainian soldiers who surrendered at the Azovstal plant in Mariupol several months ago. CNN could not independently verify either side’s claims. United Nations Secretary-General Antonio Guterres said at a news conference on Wednesday that the United Nations is seeking to set up an investigative team to study the attack following requests from both Russia and Ukraine for an investigation. He added that the terms of reference for the commission would have to be accepted by Russia and Ukraine before the fact-finding mission could begin. Video broadcast on Russian networks and shared on social media channels in Donetsk shows widespread destruction in a building and several bodies. CNN was able to locate footage from the strike in an industrial area about two miles outside the front-line town of Olenivka. The Ukrainian military said the explosion occurred on the grounds of the industrial zone, in a newly constructed building specially equipped to hold prisoners who had come out of Azovstal. The Ukrainian prosecutor’s office has launched an investigation into the strike. In a statement, it said that preliminary data indicate that “the occupying state struck the territory of penal colony No. 120 in the temporarily occupied village of Olenivka, Volnovakha district, Donetsk region.” Donetsk and Luhansk are the two regions that together form Donbass, the eastern part of Ukraine where the conflict between Ukraine and Russian-backed separatists began in 2014. The region has become the focus of Russian President Vladimir Putin’s military ambitions in Ukraine after his troops failed to capture Kyiv earlier this year. The Russian military has maintained a persistent artillery and missile barrage across the region for several weeks. The Kremlin says the goal of what it calls a “special military operation” is to take control of both Luhansk and Donetsk. CNN’s Tim Lister, Julia Kesaieva, Josh Pennington and Richard Roth contributed to this report.