Ukrainian President Volodymyr Zelensky said Monday that he believed "tens of thousands" of people in Mariupol had been killed, as he asked South Korean lawmakers to provide military assistance.

Speaking to South Korea's National Assembly by video link, Zelensky said Russia had "completely destroyed" the besieged city of Mariupol.

"The Russians completely destroyed Mariupol and burned it to ashes. At least tens of thousands of Mariupol citizens must have been killed," he said, speaking by video link to South Korean lawmakers.

"But for Russia, Mariupol is just an example. Ladies and gentlemen, we've seen a lot of destruction like this in the 20th century."

Zelensky said South Korea could help his country's fight against Russia by providing military equipment "from aeroplanes to tanks."