Clock ticking for Mariupol on Holy Thursday

A top ally of President Vladimir Putin said Russian forces will seize the last main stronghold of resistance in the besieged city of Mariupol on Thursday, after Ukraine proposed talks on evacuating troops and civilians there.

(Click here for an update to the story)

Mariupol would be the biggest city to be seized by Russia since it invaded Ukraine eight weeks ago in an attack that has taken longer than some military analysts expected, seen over five million people flee abroad and turned cities to rubble.

"Before lunchtime, or after lunch, Azovstal will be completely under the control of the forces of the Russian Federation," Ramzan Kadyrov, the head of Russia's republic of Chechnya, whose forces have been fighting in Ukraine, said of the steel plant.

Ukraine's defence ministry was not immediately available for comment. Its general staff said in an early Thursday update that missile and bomb strikes continued across the country.

The southeastern port city of Mariupol has endured the fiercest fighting of the war as besieging Russian forces try to take full control. Its capture would be a big strategic prize and would link territory held by pro-Russian separatists in the east with the Crimea region that Moscow annexed in 2014.

A few dozen civilians managed to leave the city on Wednesday in a small bus convoy, according to Reuters witnesses.

A Ukrainian marine commander, Serhiy Volny, said fighters at the steel works may not be able to hold out for much longer. President Volodymyr Zelenskiy has said an estimated 1,000 civilians are sheltering there.

Ukraine was ready for a "special round of negotiations" with no conditions "to save our guys ... military, civilians, children, the living and the wounded", negotiator Mykhailo Podolyak said on Twitter.

Kiev has proposed swapping Russian prisoners of war for safe passage for trapped civilians and soldiers. It was not known if Russia had responded to the offer of special negotiations.

Fighters remain holed up in the plant and have ignored an ultimatum by Russia to surrender.