Ukrainian and pro-Russian forces blamed each other for the deaths of at least 53 Ukrainian prisoners of war on Friday, who were killed during the shelling of a barracks where they were being held.
A further 75 captives were injured in the occupied city of Olenivka in eastern Ukraine, according to the latest information available.
Ukrainian intelligence services called the deadly attack a targeted explosion by Russian forces.
"The explosions occurred in a newly constructed building specially prepared for the Azovstal prisoners," the Ukrainian side claimed in a statement on Friday.
Azovstal is the steelworks in Mariupol where Ukrainian soldiers had holed up before submitting to Russian captivity.
Ukrainian intelligence blamed the Russian mercenary group Wagner for orchestrating the incident to destroy evidence of embezzlement during construction before an upcoming inspection, assertions the Russian Defence Ministry denied.
Sources in Ukrainian military intelligence said Russian forces had destroyed the building in the camp at night, Ukrajinska Pravda news outlet reported.
Earlier, pro-Russian separatists claimed that Ukrainian forces had shelled the barracks. "Direct hit on a barracks full of war prisoners," a separatist spokesperson told the Interfax news agency.
Kiev denied it was responsible and put the blame on Russia, calling the attack "a provocation for which Russia is responsible."
It was not possible to confirm the claims, damages and deaths independently.
Ukrainian presidential adviser Mykhailo Podoliak called the accusations a "classic, cynical and well thought-out operation under a false flag." Ukrainian military sources pointed out that it had been using high-precision HIMARS launchers in the region and that its forces do not target civilian sites.
"And especially not spots in which our imprisoned brothers in arms are possibly behind detained." Some of those imprisoned Ukrainian troops have been threatened with the death penalty for alleged war crimes. According to media reports, conditions in Olenivka's penitentiaries are inhumane.
But the separatists say Ukraine intentionally launched the strike to stop detainees - who include members of the controversial nationalist Azov regiment captured in Mariupol - from making statements.
"It was obviously an intentional targeting and a desire to destroy their representatives, including the Azov regiment, from providing testimony," said separatist leader Denis Pushilin.
An investigative committee in Moscow says it is already preparing war crimes charges because of the "attack by the Ukrainian nationalists."
Meanwhile Ukraine reduced the sentence of a Russian war criminal, from life to 15 years, for a 21-year-old who was convicted after confessing to shooting a civilian.
Olenivka, the site of Friday's deadly attack, lies in the territory claimed by the separatist groups. But social media accounts were full of photos of the city of Ilovaisk, 40 kilometres east of Olenivka, which sustained explosions overnight.
Pro-Russian separatists have been fighting with Kiev since 2014 for control of a large swathe of the country's east. The fight intensified in February, when Russia sent its own troops in, arguing it was necessary to "de-Nazify" and disarm Ukraine.
As the war enters its sixth month, Russian forces are occupying substantial swathes of eastern Ukraine, in line with Moscow's stated aims.
However, US Secretary of State Antony Blinken has told his Russian counterpart Sergei Lavrov that Russia's annexation of Ukraine would never be accepted, in the first call between the two since the war began.
Alongside discussing a possible prisoner exchange, Blinken spoke to Lavrov in light of recent statements from the Kremlin about plans to proceed with the further annexation of Ukrainian territory, he said.
He commented on Lavrov's own words about replacing the democratically elected Ukrainian government, saying: "Those plans would never be accepted."
"The world will not recognize annexations. We will impose additional significant costs on Russia if it moves forward with its plans.We'll also continue to stand with Ukraine support its ability to defend itself impose costs on Russia until it ends its aggression," Blinken said.
His comments came as North Macedonia said it would hand over several Soviet battle tanks to Ukraine as it prepares to modernize its defences, although the number was not specified.
North Macedonia, NATO's newest member, is planning to modernize its armed forces, to bring their equipment into line with Alliance standards, the Defence Ministry said.
North Macedonia still has around 30 Soviet T-72 main battle tanks. A video published by the Makfax news agency showed several of the vehicles on transport trucks as they drove through the village of Kriva Palanka near the Bulgarian border.
Meanwhile Zelensky visited Odessa on the Black Sea, amid preparations to implement an agreement it is hoped will release millions of tons grain stuck in the country's ports that could help alleviate a food crisis.
Zelensky was in the city to get a feel for the logistics of a deal that was heavily negotiated between Russia, Ukraine, Turkey and the United Nations.
