Fears are mounting for civilians trapped in the besieged cities of Mariupol, Volnovakha and Kyiv. Authorities say a Russian air strike in northeast Ukraine killed nine people on Monday, including two children.

Ukrainian officials have so far rejected Russia's proposals for evacuation corridors, pointing out that routes lead to Russia or its staunch ally Belarus, and would require people to travel through active areas of fighting.

At least 1.5 million refugees have fled Ukraine since the beginning of the invasion, the UN estimates.