Kyiv (Ukraine) (AFP) - Russia has 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.

Moscow'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 on Friday, after 21-year-old sergeant Vadim Shishimarin admitted to killing an unarmed civilian early in the offensive. The verdict is due Monday.

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.

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," President Volodymyr Zelensky said in his nightly video address late Friday, 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.

'End of the operation'

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.

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

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

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

Ukraine hopes to exchange the surrendering Azovstal soldiers for Russian prisoners. But in Donetsk, pro-Kremlin authorities are threatening to put some of them on trial. 

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".

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 $40-billion (38-billion-euro) aid package, including funds to enhance Ukraine's armoured vehicle fleet and air defence system.

And meeting in Germany, G7 industrialised nations pledged $19.8 billion to shore up Ukraine's shattered public finances.

Finland's price

The war's economic repercussions were set to expand Saturday, with Russia expected to cut off its supply of natural gas to Finland.

Finnish state energy firm Gasum said the taps would be turned off when its contract with Russia's Gazprom ended at 7:00 am (0400 GMT).

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 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.

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

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

Underground living

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.

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 Friday 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.

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