Ukrainian President Volodymyr Zelensky called Friday's shelling of a prisoner of war camp that left more than 50 people dead a "deliberate Russian war crime."
The strike on the Olenivka prison, where Ukrainian POWs were being detained in separatist-held territory in eastern Ukraine, was "a deliberate Russian war crime, a deliberate mass murder", the Ukrainian leader said.
The death toll climbed during the day on Friday with pro-Russian separatist authorities in the Donetsk region saying 53 people were killed, in an attack Moscow says was carried out by Ukrainian forces.
The Russian Investigative Committee accused Ukrainian forces of "firing on the prison where the members of the Azov battalion are being held, using American Himars system projectiles," an accusation vehemently denied by Kyiv.
The Azov regiment had distinguished itself in the defence of Mariupol, but some 2,500 Ukrainian fighters surrendered to the Russian army in May after long weeks of siege and resistance. Moscow announced the fighters would be imprisoned in Olenivka.
Ukraine says it did not target civilian infrastructure or prisoners of war, assuring that its army "fully adheres to the principles and norms of international humanitarian law."
Why does Ukraine think Russia would bomb the prison?
The Ukrainian General Staff think Russian attacked the prison, and then blamed it on Ukraine, in order to accuse Kyiv of committing war crimes and to "cover up the torture of prisoners and executions" that had been "perpetrated" there.
According to Ukrainian intelligence, the attack "was carried out by mercenaries from the Wagner division" and "was not coordinated with the leadership" of the Russian Defence Ministry, the Ukrainian General Staff said.
Russian state television broadcast images of charred barracks and destroyed metal bed frames. It showed blurred images of what appeared to be human bodies.
For its part, the European Union condemned "in the strongest possible terms the atrocities committed by the Russian armed forces and their auxiliaries."
In a statement by its head of diplomacy referring to both the bombing of the prison and accusations of torture of a Ukrainian prisoner. "These inhuman and barbaric acts constitute serious violations of the Geneva Conventions and their additional protocol and amount to war crimes," Josep Borrell added.
The statement refers to a video widely shared on Russian social media channels this week which appeared to show a Ukrainian soldier being castrated by a Russian soldier.
Russian Embassy controversy
The Russian Embassy in London added to the rhetoric around the strike on the prisoner of war camp Friday by seemingly advocating the executions of POWs, who are considered non-combatants under international law.
In a Twitter post, the diplomatic mission wrote: "Azov militants deserve execution, but death not by firing squad but by hanging, because they’re not real soldiers. They deserve a humiliating death."
Twitter put a warning on the post to say that it "violated the Twitter Rules about hateful conduct" but did not ban the account, instead saying it "may be in the public's interest for the Tweet to remain accessible."
However by Saturday morning it appeared Twitter had at least a 'shadow ban' in place which means the Embassy's feed it not so frequently served up on Twitter timelines and more difficult to find in a Twitter search.