Ukrainian troops have repulsed several Russian assaults in the country's east, British intelligence said on Monday, while President Volodymyr Zelenskiy said thousands of Russian soldiers were massing for a new offensive.
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.
"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.
The Russian invasion - the most serious conflict in Europe since the Balkans wars of the 1990s - has left a trail of death of destruction that has drawn condemnation from Western countries and triggered concern about Putin's broader ambitions.
About a quarter of Ukraine's 44 million population have been forced from their homes, cities turned into rubble, and thousands of people have been killed or injured - many of them civilians.
Austrian leader Karl Nehammer planned to meet Russian President Vladimir Putin in Moscow on Monday and will 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.
Russian forces have abandoned their attempt to capture the capital Kyiv, for now at least, but are redoubling their efforts in Ukraine's 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.
Powerful explosions rocked cities in the south and east and air raid sirens blared out across Ukraine early on Monday.
'We will answer'
Zelenskiy kept up his campaign to generate international support and rally his countrymen, warning the coming week would be important.
"Russian troops will move to even larger operations in the east of our state. They may use even more missiles against us, even more air bombs. But we are preparing for their actions. We will answer," he said in a late night video address.
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, Zelenskiy has appealed to Western powers to provide more defense help, and to punish Moscow with tougher sanctions including embargoes on its energy exports.
Zelenskiy also said Mariupol has been destroyed. Reuters journalists on Sunday saw several Russian tanks heading down a highway in the direction of the city.
The general staff of Ukraine's armed forces said it was likely the Russians would try to disrupt supply lines and strike at transport infrastructure.
Russia's defense ministry said high-precision missiles had destroyed the headquarters of Ukraine's Dnipro battalion in the town of Zvonetsky.
In a later statement, the ministry said Russian sea-launched missiles had on Sunday destroyed S-300 anti-aircraft missile systems which had been supplied to Ukraine by a European country. They systems were concealed in a hangar on the outskirts of Dnipro in central Ukraine, it said.
Reuters could not confirm the reports.
Burned alive
Austrian Chancellor Karl Nehammer said he would meet Putin on Monday in Moscow.
"We are militarily neutral, but (have) a clear position on the Russian war of aggression against #Ukraine," Nehammer wrote on Twitter. "It must stop! It needs humanitarian corridors, ceasefire & full investigation of war crimes."
Mounting civilian casualties have triggered widespread international condemnation and new sanctions.
Ludmila Zabaluk, head of the Dmytriv Village Department, north of Kyiv, said dozens of civilian bodies were found in the area.
"There were more than 50 dead people. They shot them from close distance. There's a car where a 17-year-old child was burned, only bones left. A woman had half her head blown off. A bit farther, a man lying near his car was burned alive," she said.
Reuters could not confirm the reports.
Moscow has rejected accusations of war crimes by Ukraine and Western countries. It has repeatedly denied targeting civilians in what it calls a "special operation" to demilitarize and "denazify" 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 on Monday, 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.
SocGen had faced mounting pressure to cut ties with Russia and end its more than 15-year investment in Rosbank.
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 on Sunday 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 GDP would contract by 11.2% this year due to the Western sanctions.
