UN Secretary General Antonio Guterres attends a press conference introducing the third report of the Global Crisis Response Group, examining the impact of the war in Ukraine on food, fuel, and finance, at the United Nations headquarters in New York on August 3, 2022. AFP
Kyiv and Moscow have traded blame over the strikes on the prison in Kremlin-controlled Olenivka in eastern Ukraine.
"We received the request from the Russian Federation and the request from the Ukraine" for the investigation, Antonio Guterres told a press conference.
"I decided in line with my own competencies and powers to launch a fact-finding mission," he said, noting he did not have the authority for criminal investigations.
He said the exact terms of the mission are still in the works, and hopefully Russia and Ukraine will reach an agreement on them.
"We hope to have all the facilities from both sides for access and for the obtention of all data that is necessary to be able to clarify the truths about what has happened," Guterres said, adding they are looking for "competent, independent people" to join the mission team.
The prison attack left over 50 people killed.
Moscow said that among the dead were Ukrainian forces that had laid down their arms after weeks of fighting off Russia's brutal bombardment of the sprawling Azovstal steel works in Mariupol.
Russia's defense ministry alleged that the strikes were carried out by Ukraine with US-supplied long-range missiles, in an "egregious provocation" designed to stop soldiers from surrendering.
But Kyiv has laid the blame squarely on Russia, with President Volodymyr Zelensky calling it "a deliberate mass murder of Ukrainian prisoners of war."