---

Russia declared a partial ceasefire on Saturday to allow humanitarian corridors out of the Ukrainian cities of Mariupol and Volnovakha, Russia's defence ministry said.

"From 1000 am Moscow time (0700 GMT), the Russian side declares a ceasefire and the opening of humanitarian corridors to allow civilians to leave Mariupol and Volnovakha," Russian news agencies quoted the Russian defence ministry as saying.

The announcement comes after Mariupol's mayor Vadim Boychenko said on Saturday that the city was under "blockade" by Russian forces after days of "ruthless" attacks.

While laying siege to Mariupol for days, Russian forces have cut its electricity, food, water, heating and transportation in the depths of winter, prompting comparisons to the Nazi blockade of Leningrad in World War II.

"For now, we are looking for solutions to humanitarian problems and all possible ways to get Mariupol out of the blockade," said Boychenko, calling for a ceasefire and a humanitarian corridor for food and medicine.

Since President Vladimir Putin's army invaded on Feb 24, Russia has pummelled Ukrainian cities, killed hundreds of civilians and assaulted Europe's largest atomic power plant.

The invasion has drawn condemnation and severe sanctions from Western nations balancing punishment of the Kremlin with fears of a hazardous escalation.