The President of Ukraine, Volodymyr Zelensky, has said the shelling of a prison in the separatist-controlled east holding Ukrainian servicemen was a "deliberate Russian war crime" that had claimed more than 50 lives.
"Today I received information about the attack by the occupiers on Olenivka (the prison's location), in the Donetsk region. It is a deliberate Russian war crime, a deliberate mass murder of Ukrainian prisoners of war. More than 50 dead," Mr Zelensky said.
Russia and Russian-backed separatists had earlier accused Ukrainian forces of striking the jail, saying dozens of people died and scores were wounded.
The deaths, some of which were confirmed by Reuters journalists at the prison where the men were held, overshadowed United Nations-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.
A spokesman for the separatists put the death toll at 53 and accused Ukrainian forces of targeting the prison with US-made HIMARS rockets.
Ukraine's armed forces denied responsibility, saying Russian artillery had targeted the prison to hide the mistreatment of those held there.
Foreign Minister Dmytro Kuleba said Russia had committed a war crime and called for international condemnation.
"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.
Reuters was not able to independently confirm the scene or details of the attack.
It comes as US Secretary of State Antony Blinken spoke with Russian Foreign Minister Sergei Lavrov in their first contact since the Ukraine war. Mr Blinken said he pressed Mr Lavrov to accept a proposal on freeing two Americans held in Russia.
"We had a frank and direct conversation. I pressed the Kremlin to accept the substantial proposal that we put forward," Mr Blinken told reporters.
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 illegal and unprovoked invasion of Ukraine is continuing.— Ministry of Defence 🇬🇧 (@DefenceHQ) July 29, 2022
The map below is the latest Defence Intelligence update on the situation in Ukraine - 29 July 2022
Find out more about the UK government's response: https://t.co/elQG6o2bAt
🇺🇦 #StandWithUkraine 🇺🇦 pic.twitter.com/CiUXpcjNNL
The Ukrainian 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 Ukraine 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 24 February.
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.
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, after Russia halved the flows on Wednesday citing maintenance work.
Russia, 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 it sallies say the Russian assault was entirely unprovoked.
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.
Latest Defence Intelligence update on the situation in Ukraine - 29 July 2022— Ministry of Defence 🇬🇧 (@DefenceHQ) July 29, 2022
Find out more about the UK government's response: https://t.co/CD3LU2jI92
🇺🇦 #StandWithUkraine 🇺🇦 pic.twitter.com/NMjaQWZiQU
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.
US Secretary of State Antony Blinken has spoken with Russian Foreign Minister Sergei Lavrov in their first contact since the Ukraine war, saying he pressed him to accept a proposal on freeing two Americans held in Russia.
"We had a frank and direct conversation. I pressed the Kremlin to accept the substantial proposal that we put forward," Mr Blinken told said.
The top US diplomat declined to characterise Lavrov's reaction, saying, "I can't give you an assessment of whether I think things are any more or less likely."
The proposal reportedly includes swapping basketball star Brittney Griner and former marine Paul Whelan for convicted Russian arms smuggler Viktor Bout.
Mr Blinken said he also pressed Mr Lavrov on Russia honoring a Turkish-brokered proposal to ship grain out of Ukraine and on purported plans by the Russian government to annex additional parts of Ukraine seized by Russian troops.
Mr Blinken said Russia was preparing "sham referendums" to try to "falsely demonstrate that the people in these parts of Ukraine somehow seek to become part of Russia."
He added that "the world will never recognise annexation" and that Russia would be hit by additional ramifications.