Tens of thousands of people have likely been killed in Russia's assault on the southeastern city of Mariupol, Ukrainian President Volodymyr Zelenskiy said on Monday.

"Mariupol has been destroyed, there are tens of thousands of dead, but even despite this, the Russians are not stopping their offensive," Zelenskiy said in a video address to South Korean lawmakers.

Reuters could not verify the accuracy of his estimate.

(Reporting by Pavel Polityuk; Writing by Conor Humphries)