An operation to bring civilians out of the devastated Ukrainian port city of Mariupol was underway on Monday but hundreds of people remained trapped in the Azovstal steelworks, the last stronghold of resistance to the Russian siege.

The first group of evacuees was due to arrive in a Ukrainian-held town northwest of Mariupol on Monday morning.

But people still stuck there were running out of water, food and medicine as Russian forces hemmed them into the industrial complex, whose network of bunkers and tunnels has provided shelter from weeks of Russian bombardment.

"The situation has become...