Moscow and Kyiv on Friday accused each other of bombing a jail holding Ukrainian prisoners of war in Russian-held territory, with Ukrainian President Volodymyr Zelensky saying more than 50 were killed and calling the attack a war crime.
Russia's defence ministry alleged that the strikes were carried out by Ukraine with US-supplied long-range missiles, in an "egregious provocation" designed to stop soldiers from surrendering.
It said that among the dead were Ukrainian forces that had laid down their arms after weeks of fighting off Russia's brutal bombardment of the sprawling Azovstal steel works in Mariupol.
But Zelensky laid the blame squarely on Russia.
"This was a deliberate Russian war crime, a deliberate mass murder of Ukrainian prisoners of war," Zelensky said in his daily address to the nation late Friday. "Over 50 are dead."
Zelensky said an agreement for the Azovstal fighters to lay down their arms, brokered by the United Nations and the International Committee of the Red Cross, included guarantees for their health and safety and called on those two organizations to intervene, as guarantors.
Zelensky also urged the international community, especially the United States, to have Russia officially declared as a state sponsor of terrorism.
"A decision is needed, needed right now," he said.
- Annexation 'will not be accepted' -
In a sign of Washington's continued support of Kyiv, US Secretary of State Antony Blinken spoke to his Russian counterpart Sergei Lavrov for the first time since the beginning of the war Friday, urging Moscow against annexing any more Ukrainian territory occupied by Russian forces.
"It was very important that the Russians hear directly from us that that will not be accepted -- and not only will it not be accepted, it will result in additional significant costs being imposed upon Russia if it follows through," Blinken told reporters in Washington.
Earlier in the day, Zelensky visited a port in southern Ukraine to oversee a ship being loaded with grain for export under a UN-backed plan aimed at getting millions of tonnes of Ukrainian grain stranded by Russia's naval blockade to world markets.
Ukraine's presidency released footage of Zelensky standing in front of Turkish ship Polarnet in the port of Chornomorsk on a visit to inspect grain being loaded. Ukraine's presidency said exports could start in the "coming days."
In Paris, French President Emmanuel Macron and Saudi Crown Prince Mohammed bin Salman agreed to work together to limit the impact of the war during talks, the French presidency said.
Macron took the meeting with the prince despite fierce criticism from rights groups in a bid to get major crude producer Saudi Arabia to up its production.
- Tangled beds -
Following the strike on the prison, Russian state-television showed what appeared to be destroyed barracks and tangled metal beds, but no casualties could be seen.
Ukraine's military also denied carrying out the attack saying its forces "did not launch missile and artillery strikes in the area of Olenivka settlement" in the eastern region of Donetsk.
It instead blamed Russia's invading forces for "a targeted artillery shelling" on the detention facility, saying it was being used to "accuse Ukraine of committing 'war crimes', as well as to hide the torture of prisoners and executions".
Ukraine's forces in May ended a weeks-long defence of Azovstal, with around 2,500 combatants surrendering after calling a halt to their resistance.
Moscow's state media has reported that some officers -- including those from the controversial Azov regiment -- have been taken to Russia.
Kyiv meanwhile says it has captured thousands of Russian troops during the invasion and has begun putting some on trial for alleged war crimes.
A Ukrainian court on Friday reduced the life sentence handed to a Russian soldier in May for pre-meditated murder in the country's first war crimes trial, instead jailing the serviceman for 15 years.
- Mykolaiv strikes -
Russian strikes elsewhere in Ukraine killed five people and wounded seven more on Friday in the heavily bombed city of Mykolaiv near the country's southern frontline, the regional governor said.
"They shot at another area near a public transport stop," governor Vitaliy Kim said in a statement on social media.
Mykolaiv, near the Black Sea, has seen roughly half of its estimated pre-war population of nearly 500,000 people leave and the city has been shelled daily for weeks.
It is the largest Ukrainian-controlled urban hub near the frontlines in the Kherson region, where Kyiv's army has launched a counter-offensive to regain control of the economically and strategically important coastal territory.
In a separate development, S&P Global Ratings on Friday cut Ukraine's long-term debt grade by three notches, saying a recently announced plan to defer payments means a default is "a virtual certainty."
A group of Western countries last week gave their green light to Kyiv's request to postpone interest payments on its debt and called on other creditors to do so as well.