Russia launched nighttime attacks on several cities in Ukraine, Ukrainian officials said Saturday as they and officials in Moscow blamed each other for the deaths of dozens of Ukrainian prisoners of war in a separatist-controlled area of the country's east.

Ukrainian President Volodymyr Zelenskyy said the United Nations and the International Committee of the Red Cross have a duty to react after shelling of a prison complex in Donetsk province killed the POWS.

"It was a deliberate Russian war crime, a deliberate mass murder of Ukrainian prisoners of war," Zelenskyy said in a video address late Friday. "There should be a clear legal recognition of Russia as a state sponsor of terrorism."

Both sides alleged the attack on the prison was premeditated and intended to silence the Ukrainian prisoners and to destroy evidence, including of possible atrocities.

Russia claimed Ukraine's military used U.S.-supplied precision rocket launchers to target the prison in Olenivka, a settlement controlled by the Moscow-backed Donetsk People's Republic. Separatist authorities and Russian officials said the attack killed 53 Ukrainian POWs and wounded another 75. The International Committee of the Red Cross, which has organized civilian evacuations in the war and worked to monitor the treatment of POWS held by Russia and Ukraine, said it has requested access to the prison "to determine the health and condition of all the people present on-site at the time of the attack."

"Our priority right now is making sure that the wounded receive life-saving treatment and that the bodies of those who lost their lives are dealt with in a dignified manner," the Red Cross said in a statement.

Elsewhere in eastern Ukraine, Russian rockets hit a school building in Kharkiv, the country's second-largest city, overnight, and another attack occurred about an hour later, Mayor Ihor Terekhov said Saturday.