Ukrainian President Volodymyr Zelenskiy said the shelling of a prison in the eastern Donetsk region holding Ukrainian soldiers was a "deliberate Russian war crime" that claimed more than 50 lives.
Live Briefing: Russia's Invasion Of Ukraine
RFE/RL's Live Briefing gives you all of the latest developments on Russia's full-scale invasion, Kyiv's counteroffensive, Western military aid, global reaction, and the plight of civilians. For all of RFE/RL's coverage of the war in Ukraine, click here.
Ukraine said earlier on July 29 that its security agencies had demanded that the United Nations and the International Committee of the Red Cross immediately send investigative teams to probe the killing of prisoners of war (POWs).
Zelenskiy noted that the POWs included defenders of the Azovstal plant in Mariupol, who got out of the facility in May through the mediation of the UN and the Red Cross. He said he supported the appeals from Ukrainian agencies and the ombudsman demanding action.
"Russia has proved with numerous terrorist attacks that it is the biggest source of terrorism in the world today," Zelenskiy said in his nightly video message on July 29.
Ukraine earlier launched an investigation into Russian accusations that that attack was carried out by Ukrainian forces, saying at the time that the number of Ukrainian prisoners of war killed was at least 40. It also denied the Russian accusations.
Ukrainian Foreign Minister Dmytro Kuleba called on all partners of Ukraine to condemn the shelling.
"The Russians committed another terrible war crime by shelling the correctional facility in occupied Olenivka, where they held Ukrainian prisoners of war," he said. "I call on all partners to strongly condemn this brutal violation of international humanitarian law and recognize the Russian Federation as a terrorist state."
The Russian Defense Ministry and Moscow-backed separatists in Donetsk claimed early on July 29 that Ukrainian shelling with U.S.-made high-mobility artillery rocket systems (HIMARS) was responsible for the deaths and wounding of dozens of others at the prison in Olenivka.
The television channel Rossia-1 showed what appeared to be destroyed barracks and tangled metal beds, but no casualties could be seen.
The General Staff of the Ukrainian Armed Forces said in a statement that the Russian claims were a ruse designed to hide the truth about what was happening to Ukrainians captured during the war, which is now in its sixth month, as Ukrainian forces did not fire any missiles in the area.
"The armed forces of the Russian Federation carried out targeted artillery shelling of a correctional institution in the town of Olenivka, the Donetsk region, where Ukrainian prisoners were also held," the statement said.
"The Russian enemy continues its propaganda methods of waging information warfare by accusing the Armed Forces of Ukraine of shelling civilian infrastructure and the population, while hiding its own insidious actions. Consequently, such statements about the alleged shelling of civilian infrastructure and population by the Armed Forces of Ukraine are outright lies and provocation," the statement added.
Neither side's claims about the situation could be independently verified, but Ukraine's Prosecutor-General's Office said it was opening an investigation into the incident.
"Prosecutors at the Prosecutor-General's Office have started a pretrial investigation into the fact of violation of the laws and customs of war," it said in a statement.
EU Foreign policy chief Josep Borrell condemned "in the strongest possible terms" atrocities conducted by Russian forces and their proxies in Ukraine.
"These inhumane, barbaric acts represent severe breaches of the Geneva conventions and their Additional Protocol and amount to war crimes," he said.
The International Committee of the Red Cross said it has offered support in the evacuation of the wounded and medical supplies.
"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.
It added that all prisoners of war are protected under international humanitarian law.
"They are no longer part of the fight and should not be attacked. The [International Committee of the Red Cross] has been able to visit some POWs and other detainees, but it hasn't been granted access to visit them all," the statement said.