As global outrage grew over an explosion that killed at least 50 Ukrainian prisoners held at a Russian detention camp, Ukrainian authorities called for an international investigation on Saturday while marshaling evidence that they said would prove that Russia had orchestrated what they described as a “terrorist attack.”
Since the explosion late Thursday at Correctional Colony No. 120, a prison camp in the Russian-occupied eastern region of Donetsk, the warring parties have presented diametrically opposed accounts of what happened, further embittering a war now entering its sixth month.
Russian officials claimed that Ukrainians, using precision weapons supplied by the United States, had attacked the prison themselves, to deter defectors. The Ukrainian authorities rejected the narrative as absurd and said that the deaths were a premeditated atrocity committed by Russian forces from within the prison, where survivors described being given just enough food to survive and suffering ritual beatings, including with chains and metal pipes.
The explosion is particularly painful for the government of President Volodymyr Zelensky of Ukraine because many of the dead had fought to defend Mariupol, a port on the Black Sea, and then retreated to the city’s Azovstal steelworks. For weeks there, they withstood a Russian onslaught before finally surrendering in May.
For many Ukrainians, the Azovstal siege became a symbol of the country’s suffering and defiance, and the soldiers who fought there, an estimated 2,500 of whom were taken as prisoners of war, have been viewed as heroes.
“It was a deliberate Russian war crime, a deliberate mass murder of Ukrainian prisoners of war,” Mr. Zelensky said in an address late on Friday.
Mr. Zelensky said that the Red Cross, along with the United Nations, had acted “as guarantors of the life and health of our soldiers,” and that now they must take action. “They must protect the lives of hundreds of Ukrainian prisoners of war,” he said.
A series of Russian missile strikes on civilian targets, including shopping malls and apartment buildings, has led Ukraine’s government to call on Washington to designate Moscow as a state sponsor of terrorism, something Secretary of State Antony J. Blinken has resisted.
Josep Borrell Fontelles, the European Union’s top foreign policy official, said in a statement that every day Russia’s continued “illegitimate and unjustified war of aggression” brought “further horrific atrocities,” adding that the “inhumane, barbaric acts” breached the Geneva Conventions and amounted to war crimes.
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Kaja Kallas, the prime minister of Estonia, a Baltic country that has been among Moscow’s toughest antagonists over the war, said Russia was responsible for the “mass murder” of prisoners at the camp, an act that she said called to mind “the darkest chapters of history.”
“There must be no impunity for war crimes, just like there can be no return to relations with war criminals,” she said in a statement.
For Mr. Zelensky, the prison explosion fits a pattern in which an unwarranted invasion of his country, ordered by President Vladimir V. Putin of Russia, has been accompanied by atrocities committed by Russian forces, for example in suburbs north of the capital, Kyiv, and missile strikes on civilian targets, including one this month at a shopping mall in the center of the country far from the front lines.
Russia controls around 20 percent of Ukraine’s territory, but after it deployed its superior artillery power to seize much of Luhansk Province in the eastern Donbas region earlier this month, Kyiv is now pressing a counteroffensive in Kherson Province in an attempt to reclaim land.
Moscow denies that it has committed atrocities or targeted civilians, and on Saturday the defense ministry said that Ukrainians had killed their own soldiers using precision-guided, American-made missiles, known as HIMARS, to strike the prison camp in Russian-controlled territory in eastern Ukraine.
An adviser to Ukraine’s president, Mykhailo Podoliak, told The New York Times that an expert analysis of photos and videos released by Russia indicated that the center of the explosion was inside the building, with the building’s exterior practically undamaged.
In addition, he said that the speed of Russian propaganda after the attack suggested planning. Prisoners had been moved to the barracks where the explosion occurred only days before and it was suspicious that no Russian soldiers or workers at the prison were injured, he said. Beyond that, he said that, before the explosion, Russia had moved debris to the camp from previous strikes elsewhere that had involved HIMARS weapons.
Tetiana Kravchenko, a Ukrainian rights activist whose organization has been in contact with prisoners in the camp, said that a prisoner had called his wife on Thursday night and reported hearing an explosion, rather than shelling, around 11 p.m. She said she had a recording of the call, in which the prisoner said that two of his friends had been moved to a different building within the prison on the day of the explosion and that one was now dead and the other wounded.
Soldiers held in other parts of the camp have also relayed similar accounts to their own family members, she said.
The competing claims could not be immediately independently verified. Ms. Kravchenko said she could not disclose more information without risking the safety of prisoners still being held at the camp.
At its heart, the invasion has killed tens of thousands of soldiers and civilians and brought misery to many more. It has also had far-reaching external consequences, reviving NATO, isolating Russia, raising energy prices and depressing global growth. Given Ukraine’s importance to global grain markets and Russia’s effective blockade of the country’s Black Sea ports, it has also threatened some countries in the Middle East and Africa with food shortages and hunger.
The first shipments of grain since the beginning of the conflict have been loaded onto freighters at Ukrainian ports on the Black Sea. The shipments would be the fruit of a deal brokered by Turkey and the United Nations around a week ago. Mr. Zelensky and representatives of the Group of 7 industrialized nations visited Chernomorsk, one of the three ports in Odesa Province, on Friday.
Odesa has been a frequent target of Russian missile strikes, the most recent being a week ago with an attack on the port that cast doubt on the grain deal. Indeed, Mr. Podoliak said the prison explosion was another sign that Russian assurances that it would allow safe passage for grain ships across the Black Sea could not be trusted.
Elsewhere on the ground, the battle has largely devolved into a series of incremental offensive and defensive maneuvers that have seen only limited territory changing hands each week.
The Donbas region, where Ukraine’s military said on Saturday that it repelled the latest Russian attempts to advance, is the clearest example of this slowdown. But in Kherson Province, Ukraine hopes that the HIMARS weapons and others supplied by the West will help it advance.
Ukraine said it pounded critical Russian logistical hubs overnight into Saturday and was making small but steady gains as it moved toward the city of Kherson, a shipbuilding center and port, where Ukrainian missile strikes on a bridge across the Dnipro River have left Russian defenders largely isolated.
It will be weeks and perhaps longer before the outcome of the Kherson counteroffensive will be decided, not least because the war testifies to the military maxim that attack is more difficult than defense. But a senior U.S. Defense Department official told a news briefing on Friday that there was growing evidence that steep Russian losses had left some units ill-prepared to fight. The official described Russia’s recent efforts as a failure both on the battlefield and at home.
Michael Schwirtz contributed reporting.
