Moscow will consider exchanging prisoners from Ukraine's Azov battalion for Viktor Medvedchuk, a wealthy Ukrainian businessman close to President Vladimir Putin, a Russian negotiator said today.

"We are going to study the possibility," said Leonid Slutsky, a senior member of Russia's negotiating team on Ukraine, speaking from the separatist city of Donetsk in southeastern Ukraine, the RIA Novosti news agency reported.

Mr Medvedchuk, 67, is a politician and one of Ukraine's richest people and is known for his close ties to Mr Putin. He is also a politician.

He escaped from house arrest after Russia's invasion of Ukraine in February but was re-arrested in mid-April.

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Yesterday, the Russian army announced that the last defenders of the strategic port city of Mariupol, in southeastern Ukraine, had surrendered after holding out at the Azovstal steelworks for weeks.

Among the Ukrainian fighters who gave themselves up to the Russian troops were members of the Azov regiment, a former paramilitary unit which has integrated into the Ukrainian armed forces.

Russia describes the unit, which has previous links to far-right groups, as a neo-Nazi organisation.

On 26 May, the Russian Supreme Court is scheduled to consider a request to classify the Azov regiment as a "terrorist organisation," which could complicate an exchange of these prisoners.

Denis Pushilin, leader of the Donetsk separatists, said today that the Ukrainian soldiers who defended the Azovstal plant should be put on trial.

"I believe that a legal case is inevitable: justice must prevail," said Mr Pushilin, quoted by Ria Novosti at the same news conference at which Mr Slutsky spoke.

Denis Pushilin (File image)

War can only be resolved through 'diplomacy'

The war in Ukraine can only be resolved through "diplomacy", Ukrainian President Volodymyr Zelensky has said amid a deadlock in negotiations between Kyiv and Moscow.

The war "will be bloody, there will be fighting but will only definitively end through diplomacy," he told a Ukrainian television channel.

"There are things that can only be reached at the negotiating table," he said.

"We want everything to return [to as it was before]" but "Russia does not want that," Mr Zelensky said, without elaborating.

He was speaking after Russia declared victory in its months-long operation to capture the strategic port of Mariupol after Ukraine ordered the last of its troops holed up in the city's Azovstal steelworks to lay down their arms.

'Concessions would backfire on Ukraine'

Today, Ukraine ruled out agreeing to a ceasefire with Russia and said Kyiv would not accept any deal with Moscow that involved ceding territory.

Acknowledging that Kyiv's stance on the war was becoming more uncompromising, presidential adviser Mykhailo Podolyak said making concessions would backfire on Ukraine because Russia would hit back harder after any break in fighting.

"The war will not stop (after any concessions). It will just be put on pause for some time," he told Reuters in an interview in the heavily guarded presidential office, where some of the windows and corridors are protected by sandbags.

"After a while, with renewed intensity, the Russians will build up their weapons, manpower and work on their mistakes, modernise a little, fire many generals … And they'll start a new offensive, even more bloody and large-scale."

Mr Podolyak dismissed as "very strange" calls in the West for an urgent ceasefire that would involve Russian forces remaining in territory they have occupied in Ukraine's south and east.

"The (Russian) forces must leave the country and after that the resumption of the peace process will be possible," he said.

Both sides say peace talks have stagnated. Each blames the other.

One of Europe's largest clothing market "Barabashovo", destroyed by shelling in Kharkiv

A ceasefire would play into the Kremlin's hands, Mr Podolyak said.

"They want to lock in some kind of military successes. There will definitely be no military successes given the help from our Western partners," he said.

"It would be good if the European and US elites understand to the end: Russia can't be left halfway because they will (develop) a 'revanchist' mood and be even more cruel ... They must be defeated, be subjected to a painful defeat, as painful as possible," he added.

Meanwhile, Russia's flattening of Mariupol has drawn multiple accusations of war crimes, including over a deadly attack on a maternity ward, and Ukraine has begun a legal reckoning for captured Russian troops.

The first post-invasion trial of a Russian soldier for war crimes neared its climax in Kyiv, after 21-year-old sergeant Vadim Shishimarin admitted killing an unarmed civilian early in the offensive.

The verdict is due Monday.

Mr Shishimarin told the court he was "truly sorry" but his lawyer said in closing arguments that the young soldier was "not guilty" of premeditated murder and war crimes.

Since Ukrainian forces fended off the Russian offensive around Kyiv both eastern Ukraine and Mariupol in the south have borne the brunt of a remorseless ground and artillery attack.

Smoke rises at a warehouse after shelling in Severodonetsk

The fighting is fiercest in the eastern region of Donbas, a Russian-speaking area that has been partially controlled by pro-Kremlin separatists since 2014.

"Attempts to attack Donbas continue. They completely ruined Rubizhne, Vonokvakha, just as Mariupol," Mr Zelensky said in his nightly video address yesterday, adding the Russians were "trying to do the same with Severodonetsk and many other cities".

In the eastern city of Severodonetsk, 12 people were killed and another 40 wounded by Russian shelling, the regional governor said.

Russia's forces also claimed to have destroyed a large shipment of Western-supplied weapons in northwestern Ukraine with long-range missiles.

"High-precision long-range sea-based Kalibr missiles destroyed a large batch of weapons and military equipment near the Malin railway station in Zhytomyr region delivered from the United States and European countries," the Russian defence ministry said.

'End of operation'

Mr Zelensky described the bombardment of Severodonetsk as "brutal and absolutely pointless", as residents cowering in basements described an unending ordeal of terror.

The city forms part of the last pocket of Ukrainian resistance in Lugansk, which along with the neighbouring region of Donetsk comprises the Donbas war zone.

The Russian Defence Minister declared 'the complete liberation of Mariupol'

Russia said the battle for the Azovstal steelworks - a symbol of Ukraine's dogged resistance since Russian President Vladimir Putin launched the invasion on 24 February - was now over.

Russian defence ministry spokesman Igor Konashenko said 2,439 Ukrainian personnel had surrendered at the steelworks since 16 May, the final 500 on Friday.

Defence Minister Sergei Shoigu had informed Mr Putin of "the end of the operation and the complete liberation of the (Azovstal) industrial complex and the city of Mariupol", Mr Konashenko added.

Moscow's flattening of Mariupol has drawn multiple accusations of war crimes

In Washington, Pentagon spokesman John Kirby said all prisoners of war should "be treated in accordance with the Geneva Convention and the law of war".

US President Joe Biden has cast the Ukraine war as part of a US-led struggle pitting democracy against authoritarianism.

The US Congress this week approved a $40billion (€38bn) aid package, including funds to enhance Ukraine's armoured vehicle fleet and air defence system.

And meeting in Germany, G7 industrialised nations pledged €18.75bn to shore up Ukraine's shattered public finances.

Russia cuts off natural gas supply to Finland

The war's economic repercussions continued to expand today, as Russia cut off its supply of natural gas to neighbouring Finland.

"Natural gas supplies to Finland under Gasum's supply contract have been cut off," Finnish state energy company Gasum said in a statement, adding that gas would instead be supplied from other sources via the Baltic connector pipeline, which connects Finland to Estonia.

Gasum plant in Raikkola, Imatra, Finland

Gasum a day earlier revealed the tap would be turned off when its contract with Russia's Gazprom ended at 7am.

The move, which Russia has blamed on the Nordic country's refusal to pay in rubles, comes days after Finland and Sweden submitted a joint application for NATO membership.

Moscow has repeatedly warned historically non-aligned Finland that applying for membership would be "a grave mistake with far-reaching consequences."

Both Finland and Sweden are seemingly on the fast track to joining the defence alliance, with Mr Biden offering "full, total, complete backing" to their bids.

But all 30 existing NATO members must agree on any new entrants, and Turkey has condemned the Nordic neighbours' alleged toleration of Kurdish militants.

Mr Shoigu has said the Kremlin would respond to any NATO expansion by creating more military bases in western Russia.

Saturday's halt to gas shipments follows Moscow cutting off Poland and Bulgaria last month in a move the EU described as "blackmail".

Kharkiv in artillery range

While the invasion that sparked the potential NATO expansion has ebbed around the northeastern city of Kharkiv, it remains in Russian artillery range, and hundreds of people are refusing to leave the relative safety of its metro system.

"We're tired. You can see what home comforts that we have," said Kateryna Talpa, 35, pointing to mattresses and sheets on the ground, and some food in a cardboard box.

Russian shells are seen in a damaged field in the village of Biskvitne, Kharkiv

She and her husband Yuriy are doing their best to cope in the Soviet-era station called "Heroes of Labour", alongside their cats Marek and Sima.

"They got used to it," Talpa said.

In the town of Lozova, at least eight people, including a child, were wounded yesterday when a powerful Russian missile strike gutted a newly repaired cultural centre, the largest in the region.

Kharkiv regional governor Oleg Sinegubov said all eight had been struck by shrapnel after three Russian missiles had been fired towards the 1,000-capacity building.