The United Nations chief has said that 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 US-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 defence ministry claimed in a statement on 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.