Ukrainian President Volodymyr Zelenskyy said on Monday thousands of Russian troops were massing for a new offensive in the east, and Russia said it would not halt its military operation in Ukraine for any further peace talks.

British intelligence said Ukrainian forces had already pushed back several Russian assaults in eastern regions.

Russian forces were also pushing to establish control over the southern port city of Mariupol, the lynchpin between Russian-held areas to the west and east and already devastated by weeks of siege and bombardment.

"There are tens of thousands of dead, but even despite this, the Russians are not stopping their offensive," Zelenskiy told South Korea's parliament by videolink. Reuters could not verify the accuracy of his estimate.

Zelenskyy also appealed to Seoul to provide his country with military aid to bolster its fight against the Russian onslaught.

The invasion – which Russia calls a "special military operation" – has left a trail of death and destruction that has drawn condemnation from Western countries and triggered concern about Russian President Vladimir Putin's broader ambitions.

About a quarter of Ukraine's 44 million population have been forced from their homes, cities have been turned into rubble and thousands of people have been killed or injured – many of them civilians.

Russian Foreign Minister Sergey Lavrov said the Kremlin would not halt its operation for any new round of peace talks, accusing Kyiv of failing to reciprocate in previous sessions.

Lavrov had told state television he saw no reason not to continue with talks. Although Putin had ordered a suspension of military action during a first round in February, Moscow's position had changed, he said.

"A decision was made that during the next rounds of talks, there would be no pause (in military action) so long as a final agreement is not reached," Lavrov said.

Austrian leader Karl Nehammer was due to meet Putin in Moscow on Monday and was expected to call for an end to the conflict. It would be Putin's first face-to-face meeting with a European Union leader since Russia's invasion started on Feb. 24.

"It must stop!" Nehammer wrote on Twitter.

For now at least, Russian forces have abandoned their attempt to capture the capital Kyiv, but they are redoubling their efforts in the east.

Britain's Defense Ministry said Russian shelling continued in the Donetsk and Luhansk regions. But Ukrainian forces had beaten back several assaults and destroyed Russian tanks, vehicles and artillery equipment, it said in its regular intelligence bulletin.

'We will answer'

Zelenskyy kept up his campaign to generate international support and rally his country's citizens.

Addressing South Korea's parliament, he said Russia was concentrating tens of thousands of soldiers for the next offensive. He asked Seoul for any military aid it could provide.

Since Russia invaded, Zelenskyy has appealed to Western powers to provide more defense help and to punish Moscow with tougher sanctions, including embargoes on its energy exports.

Zelenskyy also said Mariupol had been destroyed. Reuters journalists on Sunday saw several Russian tanks heading down a highway in the direction of the city.

Civilians urged to flee

Mounting civilian casualties have triggered widespread international condemnation and new sanctions.

Luhansk Governor Serhiy Gaidai, speaking to Ukrainian television on Monday, said shelling in the region was increasing day by day.

"The most difficult situation is in (the cities of) Rubizhne and Popasna. They are being shelled constantly, round the clock," Gaidai said.

He urged all civilians to evacuate. "Those that wanted to leave have already left, while now many are left in bomb shelters who are perhaps frightened to come out of the shelters, or scared to lose their possessions."

Moscow has rejected accusations of war crimes by Ukraine and Western countries. It has repeatedly denied targeting civilians and says its aim is to demilitarize and root out dangerous nationalists in its southern neighbor. Ukraine and Western nations have dismissed this as a baseless pretext for war.

German Foreign Minister Annalena Baerbock, speaking before a meeting of European ministers in Luxembourg, said Berlin saw "massive indications" of war crimes in Ukraine.

Economic cost

French bank Societe Generale became the latest company to retreat from Russia, agreeing to sell its stake in Rosbank and the Russian lender’s insurance subsidiaries to Interros Capital, a firm linked to billionaire Vladimir Potanin.

The Russian invasion has triggered a barrage of financial sanctions from the United States, Europe and Britain, prompting Western companies to sell their Russian assets.

Several EU ministers said on Monday the bloc's executive was drafting proposals for an oil embargo on Russia, although there was still no agreement to ban Russian crude.

The World Bank forecast the war would cause Ukraine's economic output to collapse by 45% this year, with half of its businesses shuttered, grain exports mostly cut off by Russia's naval blockade and destruction rendering economic activity impossible in many areas.

The bank forecast Russia's gross domestic product (GDP) would contract by 11.2% this year due to the Western sanctions.