Reuters could not immediately verify the accounts of either side in the conflict.

Video released by a Russian media war correspondent, Andrei Rudenko, showed Russian-backed military personnel sifting through the burned out remains of what he said was the prison. The smashed roof of the building was hanging down.

The charred remains of bodies could be seen. One body was blackened with flames. More bodies lay outside, covered with sheets. Flies swarmed around bloody rags.

Reuters was not able to independently confirm the scene.

“A missile strike from the U.S.-made multiple launch rocket system (HIMARS) was carried out on a pre-trial detention center in the area of the settlement of Olenivka, where Ukrainian military prisoners of war, including fighters from the Azov battalion, are being held,” the Russian defence ministry said.

Russian-backed separatist leader Denis Pushilin was quoted by Interfax as saying that the prison, located in the frontline town of Olenivka, housed 193 people and that there were no foreigners among the detainees.

The General Staff of the Ukrainian armed forces denied Ukraine carried out the attack, and accused Russia of shelling the prison in order to falsely incriminate Ukraine of war crimes and to cover up executions.

“The armed forces of the Russian Federation carried out a targeted artillery shelling of a correctional institution in the settlement of Olenivka, Donetsk oblast, where Ukrainian prisoners were also held,” the General Staff of the Ukrainian armed forces said in a statement.

“In this way, the Russian occupiers pursued their criminal goals – to accuse Ukraine of committing ‘war crimes’, as well as to hide the torture of prisoners and executions which they carried out there.”

The Kyiv government has accused Russia of atrocities and brutality against civilians during the invasion and said it has identified more than 10,000 possible war crimes. Russia has denied targeting civilians or involvement in war crimes and accused Kyiv of staging them to smear its forces.

United States and more than 40 other countries agreed on July 14 to coordinate investigations into suspected war crimes in Ukraine, with the bulk of the claims concerning alleged actions by Russian forces and their proxies. A week ago, Russia announced plans to investigate war crimes it claims have been committed by Ukrainian forces.

After Russia took control over the southern port of Mariupol following a months-long siege of the Azovstal steel plant there, many Ukrainian fighters including some from the Azov battalion were taken prisoner in Olenivka following their surrender.

(Reporting by ReutersEditing by Guy Faulconbridge and Frances Kerry)