(Adds Mariupol scene)
* Toll from strike last week shows Russia can inflict huge losses
* Russian mine clearers sweeping captured Azovstal plant
* Ukraine says its forces repel assault on Sievierodonetsk
* Russia forces control largely unbroken swathe of east and south
By Pavel Polityuk and Natalia Zinets
KYIV/MARIUPOL, May 23 (Reuters) - Ukrainian President Volodymyr Zelenskiy told global business leaders on Monday the world must increase sanctions against Russia to deter other countries from using "brute force" to achieve their aims.
Zelenskiy spoke via video link to the World Economic Forum in Davos as the Ukrainian military claimed to have held off a Russian assault on Sievierodonetsk, an eastern city that has become the main target of a Russian offensive after the surrender of the southern port city of Mariupol last week.
Zelenskiy also revealed Ukraine's worst military losses from a single attack of the war on Monday, saying 87 people had been killed last week when Russian forces struck a barracks housing troops at a training base in the north.
Previously, Kyiv had said eight people died in the May 17 strike on the barracks in the town of Desna.
In the first of what could be many war crimes trials arising from Russia's Feb. 24 invasion, a court in Kyiv sentenced a young Russian tank commander to life in prison for killing an unarmed civilian.
Ukraine Prosecutor General Iryna Venediktova told the Washington Post about 13,000 cases of Russian alleged war crimes were being investigated.
Russia has denied targeting civilians or involvement in war crimes while it carries out what it calls a "special military operation" in Ukraine.
STARBUCKS TO GO
With the conflict about to enter its fourth month, Zelenskiy urged countries to put more pressure on Moscow and accused them of not exhausting sanctions.
"The sanctions should be maximum, so that Russia - and every other potential aggressor who wants to wage a brutal war against its neighbour - clearly knows the immediate consequences of their actions," he told the Davos meeting.
He demanded an oil embargo, the blockage of all Russian banks and termination of all trade. Foreign businesses should withdraw completely from Russia and the Russian IT industry should be cut off from the West, he said.
The United States, the European Union and other entities have already imposed broad sanctions on Russia, and on Monday Starbucks Corp became the latest Western brand to announce it was pulling out of the country. The Seattle-based coffee chain has 130 stores in Russia. Its decision follows a similar move by McDonald's. The fast food chain's trademark "Golden Arches" were lowered near Moscow on Monday.
Russia has focused its offensive on the eastern Donbas region since its troops were driven out of the area around the capital Kyiv and the north at the end of March.
Having captured Mariupol last week after a three-month siege, Russian forces now control a largely unbroken swathe of the east and south, freeing up more troops to join the main Donbas fight.
Moscow is trying to encircle Ukrainian forces and fully capture the Luhansk and Donetsk provinces that make up the Donbas and where Moscow backs separatist forces.
Kyiv said on Monday Russian forces had tried to storm Sievierodonetsk but were unsuccessful and retreated. The city lies in the easternmost part of a Ukrainian-held pocket of the Donbas and one of the last areas of Luhansk still outside Russia's grip.
Luhansk governor Serhiy Gaidai said Russia was "wiping Sievierodonetsk from the face of the earth" and trying to advance from three directions: to overrun Sievierodonetsk, cut off a highway south of it and cross the river further west.
WAR CRIMES TRIAL
At the trial in Kyiv, Judge Serhiy Agafonov said 21-year-old tank commander Vadim Shishimarin, carrying out an order by a soldier of higher rank, had fired several shots at the victim's head from an automatic weapon.
Shishimarin pleaded guilty to killing 62-year-old Oleksandr Shelipov in the northeastern Ukrainian village of Chupakhivka on Feb. 28, four days after Russian troops rolled over the border.
Shishimarin, standing in a reinforced glass box in the courtroom, showed no emotion as the verdict was read out.
The trial, which began only last week, has huge symbolic significance for Ukraine, which has accused Russia of atrocities and widespread brutality against civilians
The Kremlin did not comment on the verdict. It has previously said that it has no information about the trial.
The head of the Moscow-backed separatists in Donetsk, Denis Pushpin, said Ukrainian prisoners of war captured at the Azovstal steel-works in Mariupol could also face tribunals.
"Now, they are being kept on the territory of the Donetsk People's Republic. It is planned to organizer an international tribunal later."
Ukraine has been trying to secure a prisoner swap for the fighters who surrendered last week in Mariupol. A Russian deputy foreign minister was quoted as saying Moscow could discuss a swap.
In other developments, a Russian diplomat at the country's permanent mission at the United Nations in Geneva said he was leaving his post because of his disagreement with Moscow's invasion of Ukraine, a rare political resignation over the war.
Boris Bondarev, who identified himself on LinkedIn as a counsellor who worked on arms control, told Reuters: "The scale of this disaster drove me to do it."
"I simply cannot any longer share in this bloody, witless and absolutely needless ignominy."
At a cemetery outside Mariupol, a grief-stricken mother named Natalya sobbed inconsolably on Sunday as she mourned her only son.
Vladimir Voloshin, 28, was killed on March 26 when shrapnel smashed into his skull and chest in the fight for the city.
The 57-year-old Natalya said he had just graduated from a naval academy when Ukraine announced a general mobilisation to counter the Russian invasion.
"He had been supposed to set sail in February," she said between sobs. "But then the war started. For no reason at all."
(Reporting by Pavel Polityuk and Natalia Zinets in Kyiv and Reuters journalists in Mariupol Writing by Peter Graff and Angus MacSwan; Editing by Nick Macfie and Frank Jack Daniel)