Russia claims that Ukraine attacked a prison with Ukrainian prisoners of war from Mariupol.
The Ukrainian military counters, saying Russia shelled the prison to cover up torture and executions. 50 people are said to have been killed and over 100 injured.
Russia and Russian separatists with close ties to Moscow claim that around 40 Ukrainian prisoners of war captured during fighting in Mariupol were killed after Ukrainian artillery hit a prison in the Russian-controlled town of Oljonivka in Donetsk on Friday.
Denying the charges
Ukraine denies the accusations, saying it never attacks civilian targets and that it was Russia that deliberately attacked the building.
But no version of the incident has been verified.
In the player above: Images show the devastation after the attack.