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, Ukraine 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 February 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 said 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.
With the conflict about to enter its fourth month, Zelenskiy urged countries to put more pressure on Russia 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.
US Secretary of Defense Lloyd Austin said on Monday that 20 countries have approved weapons shipments to Ukraine.
Greece, Italy and Poland had all agreed to send artillery systems, Austin said, while Denmark had promised missiles and assistance with training had been promised by several others.
Austin added that the group of countries supporting Ukraine with arms had grown since its initial meeting last month in Germany, with Austria, Bosnia-Herzegovina, Colombia and Ireland now also participating, bringing its membership to 47.
A next meeting has been set for June 15, on the sidelines of a meeting of NATO defence ministers.
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.
Russia is trying to encircle Ukrainian forces and fully capture the Luhansk and Donetsk provinces that make up the Donbas and where it backs separatist forces.
Ukraine said 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.
The head of the Russian-backed separatists in Donetsk, Denis Pushpin, said on Monday 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 organise an international tribunal later."
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 the 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."
with reporting from DPA
