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.

Reuters could not verify the accuracy of his estimate. Zelensky 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 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.

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.

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.

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.