STORY: "I am appealing especially to the United States of America. A decision is needed and it is needed now," he said in a late night video address calling for legal recognition that Russia was a state sponsor of terrorism.
Dozens of Ukrainian prisoners of war appeared to have been killed when a prison building was destroyed in a missile strike or explosion, with Moscow and Kyiv accusing each other of responsibility.
The deaths, some of which were confirmed by Reuters journalists at the prison where the men were held in eastern Donetsk province, overshadowed a U.N.-brokered deal to restart shipping grain from Ukraine and ease a worldwide food crisis.
Forty prisoners were killed and 75 wounded at the prison in the frontline town of Olenivka held by Moscow-backed separatists, Russia's defence ministry said.
A spokesman for the separatists put the death toll at 53 and accused Kyiv of targeting the prison with U.S.-made HIMARS rockets.
Ukraine's armed forces denied responsibility, saying Russian artillery had targeted the prison to hide the mistreatment of those held there. Foreign Minister Dmytro Kuleba said Russia had committed a war crime and called for international condemnation.