Moscow claimed Sunday its troops had "entirely" besieged the southern Ukrainian city of Kherson and the city of Berdyansk in the southeast, as the Russian army pressed ahead with the invasion of the pro-Western country.

"Over the past 24 hours, the cities of Kherson and Berdyansk have been completely blocked by the Russian Armed Forces," Russian Defense Ministry spokesperson Igor Konashenkov said in a statement carried by Russian news agencies.

Russian troops captured a strategically important former airfield in the south of Ukraine Saturday, and could head toward Mariupol from there during the third day of fighting.

Mariupol is close to the eastern Ukrainian separatist areas and is the last major port under government control on the Sea of Azov.

Ukrainians are fighting hard against Russian forces but tens of thousands of Russian troops have now entered Ukraine, according to an estimate by Ukrainian President Volodymyr Zelenskyy.