RUSSIA and Ukraine accused each other today of shelling a prison in a separatist region of eastern Ukraine, an attack that reportedly killed dozens of Ukrainian prisoners of war.

Both sides said the assault had been premeditated with the aim of covering up atrocities.

Russia claimed that Ukraine's military had used US-supplied multiple rocket launchers to strike the prison in Olenivka, a settlement in the Moscow-backed Donetsk People’s Republic.

Separatist authorities and Russian officials said that the attack had killed 53 Ukrainian prisoners of war and wounded another 75.

Moscow opened a probe into the attack, sending a team to the site from the Investigative Committee, Russia’s main criminal investigation agency.

The state-run RIA Novosti news agency reported that fragments of US-supplied precision rockets had been found at the site.

The Ukrainian military denied carrying out any rocket or artillery strikes on Olenivka and accused the Russians of shelling the prison to cover up the alleged torture and execution of Ukrainians there.

An adviser to Ukrainian President Volodymyr Zelensky described the attack as a “a deliberate, cynical, calculated mass murder of Ukrainian prisoners.”

Neither claim could be independently verified.

Donetsk’s deputy military commander, Eduard Basurin, suggested that Ukraine had decided to strike the prison to prevent captives from revealing key military information.

Ukraine “knew exactly where they were being held and in what place,” he said. “After the Ukrainian prisoners of war began to talk about the crimes they committed and orders they received from Kiev, a decision was made by the political leadership of Ukraine: carry out a strike here.”

Ukrainian presidential adviser Mykhailo Podolyak called for a “strict investigation” of the deadly attack and urged the United Nations and other international organisations to condemn it.

He said the Russians had transferred some Ukrainian prisoners to the building that was hit just a few days before the strike, suggesting that it had been planned in advance.

“The purpose — to discredit Ukraine in front of our partners and disrupt weapons supply,” he tweeted.

Elsewhere, Mr Zelensky’s office said that at least 13 civilians had bee killed and another 36 wounded by Russian shelling over the last 24 hours.

In the capital Kiev, an appeals court reduced to 15 years the life sentence of a Russian soldier convicted of killing a civilian.

Critics had said the sentencing of Vadim Shishimarin, 21, was unduly harsh, given that he had confessed to the crime and expressed remorse.