The Russians had earlier given a "surrender or die" deadline of Sunday mid-day to around 2,500 Ukrainian fighters who are resisting the Russian siege of the city from a steel plant with a network of underground passages in the last pocket of resistance in Mariupol.

The Russian military had announced that those who laid down their weapons were “guaranteed to keep their lives” but the Ukrainians did not submit, just as they rejected previous ultimatums.

Ukraine's Prime Minister Denys Shmyhal told US news channel ABC, "We will fight absolutely to the end, to the win, in this war."?

Shmyhal added Ukraine is prepared to end the war through diplomacy, if possible, but they do not have intention to surrender.

Seizing Mariupol would free up Russian forces to join an expected all-out offensive for control of the Donbas region in the country's east. The region is the industrial part of Ukraine that has renewed Russian focus after their forces were withdrawn from around Kyiv and the country's north to redeploy eastward.

The siege of Mariupol and its relentless bombardment by Russians have come at a terrible cost. Ukrainian officials have said that around 21,000 people, including hundreds of children, have been killed and up to 90 per cent of the city has been destryed.

The Russian defence ministry has said, "All those who will continue resistance will be destroyed."

Their statement added that they intercepted communications that indicated there were about 400 foreign mercenaries along with Ukrainian troops at the Azovstal steel mill, a claim that could not be independently verified.

Ukrainian Deputy Defence Minister Hanna Malyar described Mariupol as a “shield defending Ukraine” as Russian troops prepare for the battle in the Donbas, where Moscow-backed separatists already control some territory.

Malyar said the Russians continued to hit Mariupol with airstrikes and could be getting ready for an amphibious landing to beef up their ground forces.

Capturing the city would be Russia's biggest victory after two months of costly fighting and could help reassure the Russian public amid the worsening economic situation from Western sanctions.

It would allow Russia to secure a land corridor to the Crimean Peninsula, which it annexed from Ukraine in 2014, and deprive Ukraine of a major port and its prized industrial assets.

Mariupol's seizure also would make more troops available for the offensive in the east, which, if successful, would give Russian President Vladimir Putin a vital piece of the country and a badly needed victory that he could sell to the Russian people.

Tunnels at the sprawling Azovstal steel mill, which covers an area of more than 11 square km (over 4.2 square miles), have allowed the defenders to hide and resist.

Ukrainian President Volodymyr Zelenskyy said the fall of Mariupol could scuttle any attempt at a negotiated peace. He said in an interview, “The destruction of all our guys in Mariupol - what they are doing now - can put an end to any format of negotiations."

In his nightly address to the nation, Zelenskyy called on the West to send more heavy weapons immediately if there is any chance of saving the city, adding Russia is “deliberately trying to destroy everyone who is there”.

In a reminder that no part of Ukraine is safe until the war ends, Russian forces carried out new missile strikes on Sunday near Kyiv and elsewhere in an apparent effort to weaken Ukraine's military capacity before the anticipated assault in the east.

After the humiliating loss of the flagship of its Black Sea Fleet to what the Ukrainians said was a missile attack, Russia's military command vowed on Friday to step up strikes on Kyiv. Russia said on Sunday that it had attacked an ammunition plant near Kyiv overnight with precision-guided missiles, the third such strike in as many days.

The Russian military also claimed to have destroyed Ukrainian air defense radar equipment in the east, near Sievierodonetsk, as well as several ammunition depots elsewhere.

Explosions were reported overnight in Kramatorsk, an eastern city where rockets earlier this month killed at least 57 people at a train station crowded with civilians trying to evacuate ahead of the expected Russian offensive.

A regional official in eastern Ukraine said at least two people were killed Sunday when Russian forces fired at residential buildings in the town of Zolote, near the front line in the Donbas.

With AP?inputs