Yevgeny Prigozhin, leader of Russia's Wagner Group mercenary force, said they would leave on May 10 because of heavy losses and inadequate ammunition supplies.
Russia continues attacking the besieged city of Bakhmut in the eastern Donetsk region. Meanwhile, a report said Ukraine is rethinking its counter-offensive strategy after the leak of US docuemnts.
Zelenskyy has extended invitations to Xi before in recent months, but this explicit call to visit comes days after the Chinese leader visited Putin in Russia last week.
Russia-Ukraine war: President Vladimir Putin has threatened to use all available means to defend his land, a remark many have interpreted to include nuclear weapons.