Dozens of Ukrainian prisoners of war appeared to have been killed in a missile strike on Friday, with Moscow and Kyiv accusing each other of carrying out the attack.
The incident overshadowed UN-backed efforts to restart grain shipments from Ukraine and ease a looming global hunger crisis stemming from the war, now in its sixth month.
Russia's defence ministry said 40 prisoners were killed and 75 wounded in the attack on the prison in the frontline town of Olenivka, in a part of Donetsk province held by separatists.
It accused Kyiv of targeting it with US-made HIMARS rockets, Russian news agencies reported.
Ukraine's armed forces denied carrying out the strike and blamed it on Russian forces, saying Russian artillery had targeted the prison.
"In this way, the Russian occupiers pursued their criminal goals - to accuse Ukraine of committing 'war crimes', as well as to hide the torture of prisoners and executions," the general staff of the Ukrainian Armed Forces said.
The Russian defence ministry said the prison housed Ukrainian prisoners of war and that eight prison staff were also wounded. Russian-backed separatist leader Denis Pushilin was quoted as saying there were no foreigners among 193 people held there.
Video released by Russian war correspondent Andrei Rudenko showed Russian-backed military personnel sifting through the burned-out remains of what he said was the prison.
The smashed roof of the building was hanging down and the charred remains of bodies could be seen.
Reuters was not able to independently confirm the scene or details of the attack.
Separately, Ukraine said at least five people had been killed and seven wounded in a Russian missile strike on the southeastern city of Mykolaiv, a river port just off the Black Sea, as Russia fired across frontlines in eastern and southern Ukraine.
A missile struck near a public transport stop, regional governor Vitaly Kim said on Telegram.
Russia, which denies targeting civilians, did not immediately comment on the situation and Reuters could not verify the battlefield reports.
The Kyiv government has accused Russia of atrocities and brutality against civilians during the invasion and said it has identified more than 10,000 possible war crimes.
Russia has denied involvement in war crimes, accused Kyiv of staging them to smear its forces and said it is investigating Ukrainian war crimes.
Russia and Ukraine agreed last week to unblock grain exports from Black Sea ports, which have been threatened by Russian attacks since it invaded Ukraine on Feb 24.
The deal was the first diplomatic breakthrough of the conflict and wheat prices being offered in Asia slid this week on expectations of higher supplies.
But fierce fighting makes it extremely risky.
Ukraine is ready to start shipping grain from two ports under the UN-brokered agreement but no date has been set for the first shipment, Ukrainian Infrastructure Minister Oleksandr Kubrakov told reporters in the southern port of Odesa.
"In total, 17 ships were loaded before the war. Today we began the loading of another ship at Chornomorsk. We have, in principle, solved almost all technical questions," he said.
He said he hoped the first ships could leave port before the end of the week, while Bridget Brink, the US ambassador to Ukraine, said she hoped agreement would reached later on Friday to open the way for the first shipment.
United Nations aid chief Martin Griffiths said on Thursday the world body hoped poorer countries would get priority, citing Somalia, where nearly a quarter of a million people face starvation.
While the blockage of grain in Ukraine, one of world's biggest exporters, has fed into food price rises around the world, shortages of Russian gas have raised energy prices in Europe and prompted fears of shortages over winter.
Russian gas flows via the Nord Stream 1 pipeline to Germany remained at just 20% of capacity on Friday, after Russia halved the flows on Wednesday citing maintenance work.
Moscow, which describes its invasion of Ukraine as a "special military operation" conducted in self-defence, blames Western sanctions for the low gas supplies. Ukraine and its allies say the Russian assault was entirely unprovoked.
FOCUS ON SOUTH
An intelligence update from Britain said Russia has ordered mercenaries to hold sections of the frontline in Ukraine - a sign it is running short of combat infantry as Kyiv steps up a counter-offensive in the south.
Greater reliance on fighters from the Russian private military company Wagner Group for frontline duties rather than their usual work in special operations would be another sign that Russia's military is under stress.
"This is a significant change from the previous employment of the group since 2015, when it typically undertook missions distinct from overt, large-scale regular Russian military activity," the ministry said.
Wagner and the Kremlin were not available for comment.
Officials in Kyiv said on Wednesday they had observed a "massive redeployment" of Russian forces to the south where British defence officials believe Russia's 49th Army, stationed on the west bank of the Dnipro River, is vulnerable.
Ukraine's counter-attacks in the south come as Russia battles for control of the entirety of the industrialised Donbas region in the east, comprising the provinces of Donetsk and Luhansk.