Ukrainian President Volodymyr Zelensky 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 repulsed 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," Zelensky told South Korea's parliament by videolink.
Zelensky also appealed to Seoul to provide his country with military aid to bolster its fight against the Russian onslaught.
Russian Foreign Minister Sergei 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 told state television he saw no reason not to continue with talks. But although Russian President Vladimir 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 February 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 defence 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.
Zelensky kept up his campaign to generate international support and rally his countrymen.
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, Zelensky has appealed to Western powers to provide more defence help, and to punish Moscow with tougher sanctions, including embargoes on its energy exports. Zelenskyy also said Mariupol had been destroyed.
Russia's defence 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. The systems were concealed in a hangar on the outskirts of Dnipro in central Ukraine, it said.
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 demilitarise and root out dangerous nationalists in its southern neighbour. 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.
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 45pc 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 11.2pc this year due to the Western sanctions.
On Sunday night, Zelensky warned that the coming week would be as crucial as any during the war, saying “Russian troops will move to even larger operations in the east of our state.”
Zelensky also said that Ukraine’s fate as the war shifts south and east depends on whether the United States will help match a surge in Russian weaponry in the regions.
“To be honest, whether we will be able to (survive) depends on this,” said Zelenskyy said in a “60 Minutes” interview that aired Sunday night, speaking through a translator. “Unfortunately, I don’t have the confidence that we will be receiving everything we need.
Zelenskyy said he was grateful to Biden for U.S. military aid to date but added that he “long ago” forwarded a list of specific items Ukraine desperately needed and that history would judge Biden’s response.
“He has the list,” Zelensky said. “President Biden can enter history as the person who stood shoulder to shoulder with the Ukrainian people who won and chose the right to have their own country. (This) also depends on him.”
Experts have said that the next phase of the battle may begin with a full-scale offensive. The outcome could determine the course of the conflict, which has flattened cities, killed untold thousands and isolated Moscow economically and politically.
Questions remain about the ability of Russia’s depleted and demoralized forces to conquer much ground after their advance on the capital, Kyiv, was repelled by determined Ukrainian defenders. Britain’s Defense Ministry reported Sunday that the Russian forces were trying to compensate for mounting casualties by recalling veterans discharged in the past decade.
In his Sunday night address, Zelensky also accused Russia of trying to evade responsibility for war crimes in Ukraine.
“When people lack the courage to admit their mistakes, apologize, adapt to reality and learn, they turn into monsters. And when the world ignores it, the monsters decide that it is the world that has to adapt to them,” Zelensky said.
“The day will come when they will have to admit everything. Accept the truth,” he added.
In Washington, a senior U.S. official said that Russia has appointed Gen. Alexander Dvornikov, one of its most seasoned military chiefs, to oversee the invasion. The official was not authorized to be identified and spoke on condition of anonymity.
Until now, Russia has had no central war commander on the ground.
The new battlefield leadership comes as the Russian military prepares for what is expected to be a large, focused push to expand control in Ukraine's east. Russia-backed separatists have fought Ukrainian forces in the eastern Donbas region since 2014 and declared some territory there as independent.
Dvornikov, 60, gained prominence as head of the Russian forces deployed to Syria in 2015 to shore up President Bashar Assad’s government during the country's devastating civil war. U.S. officials say he has a record of brutality against civilians in Syria and other war theaters.
Russian authorities do not generally confirm such appointments and have said nothing about a new role for Dvornikov, who received the Hero of Russia medal, one of the country’s highest awards, from President Vladimir Putin in 2016.
U.S. national security adviser Jake Sullivan, speaking Sunday on CNN's “State of the Union," played down the significance of the appointment.
“What we have learned in the first several weeks of this war is that Ukraine will never be subjected to Russia,” Sullivan said. “It doesn’t matter which general President Putin tries to appoint.”
Western military analysts say Russia's assault has increasingly focused on a sickle-shaped arc of eastern Ukraine — from Kharkiv, Ukraine’s second-largest city, in the north to Kherson in the south.
The narrower effort could help Russia's problem, earlier in the war, of spreading its offensive too widely over too great a geographic area.
“Just looking at it on a map, you can see that they will be able to bring to bear a lot more power in a lot more concentrated fashion,” by focusing mainly on eastern Ukraine, Pentagon spokesman John Kirby said Friday.
Newly released Maxar Technologies satellite imagery showed an 8-mile (13-kilometer) convoy of military vehicles headed south through Ukraine to Donbas, recalling images of a convoy that got stalled on roads to Kyiv for weeks before Russia gave up on trying to take the capital.
The president of the European Commission said on CNN’s “State of the Union” Sunday that Ukraine's response to a questionnaire she recently handed to Zelenskyy will enable her to decide whether to recommend the nation as a candidate to join the EU.
The process normally takes years, but Ursula von der Leyen has said Ukraine’s application could take just weeks to consider.
“Yesterday, somebody told me: ‘You know, when our soldiers are dying, I want them to know that their children will be free and be part of the European Union,'” von der Leyen said.
Ukrainian authorities have accused Russian forces of committing war crimes against civilians, including airstrikes on hospitals, a missile attack that killed at least 57 people at a train station and other violence discovered as Russian soldiers withdrew from the outskirts of Kyiv.
The Institute for the Study of War, an American think tank, predicted that Russian forces will “renew offensive operations in the coming days” from Izyum, a town southeast of Kharkiv, in the campaign to conquer the Donbas, which comprises Ukraine's industrial heartland.
But in the view of the think tank's analysts, “The outcome of forthcoming Russian operations in eastern Ukraine remains very much in question.”