UNITED NATIONS (AP) — The United Nations chief said Wednesday he is appointing a fact-finding mission in response to requests from Russia and Ukraine to investigate the killings at a prison in a separatist region of eastern Ukraine that the warring nations accuse each other of carrying out.

Secretary-General Antonio Guterres told reporters he doesn’t have authority to conduct criminal investigations but does have authority to conduct fact-finding missions, and the terms of reference for a mission to Ukraine are currently being prepared and will be sent to the governments of Ukraine and Russia for approval.

Russia claimed that Ukraine’s military used U.S.-supplied rocket launchers to strike the prison in Olenivka, a settlement controlled by the Moscow-backed Donetsk People’s Republic. Separatist authorities and Russian officials said the attack killed 53 Ukrainian POWs and wounded another 75.

The Ukrainian military denied making any rocket or artillery strikes in Olenivka. The intelligence arm of the Ukrainian defense ministry claimed in a statement Wednesday to have evidence that local Kremlin-backed separatists colluded with the Russian FSB, the KGB’s main successor agency, and mercenary group Wagner to mine the barrack before “using a flammable substance, which led to the rapid spread of fire in the room.”

The Ukrainian military on Tuesday likewise claimed that the barrack had been blown up from the inside, citing the nature of damage which it said was inconsistent with Russian claims that Ukraine had shelled the building. It was not immediately possible to verify these claims.

Secretary-General Guterres said he took the requests from Russia and Ukraine for a U.N. investigation of last Friday’s deadly incident “very seriously” and expressed hope that both countries will agree to the terms of reference. At the same time, he said, the U.N. is looking for “competent, independent people” to take part in the mission.

The U.N. chief also expressed hope the warring countries will facilitate the mission’s access and provide the data required “to clarify the truth about what happened.”

The Ukrainian POWS at the Donetsk prison included troops captured during the fall of Mariupol. They spent months holed up with civilians at the giant Azovstal steel mill in the southern port city. Their resistance during a relentless Russian bombardment became a symbol of Ukrainian defiance against Russia’s aggression.

More than 2,400 soldiers from the Azov Regiment of the Ukrainian national guard and other military units gave up their fight and surrendered under orders from Ukraine’s military in May.

Scores of Ukrainian soldiers have been taken to prisons in Russian-controlled areas. Some have returned to Ukraine as part of prisoner exchanges with Russia, but other families have no idea whether their loved ones are still alive, or if they will ever come home.

Ukraine’s defense ministry claimed Wednesday that Ukrainian captives at the prison had been subject to “bullying, physical humiliation, and psychological demoralization” in an attempt to coerce them into starring in pro-Russian propaganda videos.

“Ukrainian prisoners showed exceptional courage and invincible willpower,” the ministry said, alleging that Moscow and the separatists did not intend to include the captives in an exchange, and opted to “deliberately destroy” them, in order to hide signs of torture which could serve as evidence in international criminal proceedings. It did not immediately disclose how it had arrived at this assessment.

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