Ukraine's presidential adviser Mykhailo Podolyak has posted on Twitter that the Russians are attacking the Azovstal steel plant in the port city of Mariupol with powerful "bunker-buster" bombs.

The bombs penetrate thick concrete before exploding.

Podolyak appealed for efforts to evacuate civilians sheltered at the plant.

He tweeted, "The world watches the murder of children online and remains silent. Religious and world leaders can stop it by organizing humanitarian corridors."