Denys Shmyhal
Denys Shmyhal Ukrainian Governmental Press Service/Handout via REUTERS

Ukraine on Sunday vowed to fight to the end in Mariupol, after a Russian ultimatum expired for remaining forces to surrender in port city where Moscow is pushing for a major strategic victory, AFP reports.

"The city still has not fallen," Prime Minister Denys Shmyhal said hours after Moscow's deadline had passed for fighters holed up and surrounded in a sprawling fortress-like steelworks to surrender.

"There's still our military forces, our soldiers. So they will fight to the end," he stated in an interview with ABC's "This Week".

Moscow has shifted its military focus to gaining control of the eastern Donbas region and forging a land corridor to already-annexed Crimea.

Russia's defense ministry said Sunday there were up to 400 mercenaries inside the encircled Azovstal steel plant, calling on Ukrainian forces inside to "lay down their arms and surrender in order to save their lives".

Moscow claims Kyiv has ordered fighters of the nationalist Azov battalion to "shoot on the spot" anyone wanting to surrender.

Ukraine President Volodymyr Zelenskyy warned that if Russian forces killed the remaining troops defending the city, it would end the peace talks.

Russian President Vladimir Putin has already said the talks are at a "dead end".

Shmyhal said on Sunday Ukraine wanted a diplomatic solution but would fight to the end if necessary.

"We will not surrender," he stressed, adding that while several large cities are under siege, not one -- with the exception of Kherson in the south -- had fallen, and more than 900 towns and cities had been liberated.

Zelenskyy, meanwhile, warned on Sunday that Russia might use chemical or even nuclear weapons and urged world leaders to prepare for such contingencies.

Speaking with CNN in an interview, Zelenskyy said the potential use of chemical or nuclear weapons posed a threat to the entire planet, and urged foreign powers to prepare anti-radiation medicine and air raid shelters.

“Not only me – all of the world, all of the countries have to be worried because it can be not real information, but it can be truth,” he stated.

Russia recently claimed that Ukraine is running chemical and biological labs with US support.

White House press secretary Jen Psaki later rejected the claims from Russia as "false" and "preposterous," and warned they could serve as a pretext for the Russians to deploy chemical weapons in their assault on Ukraine.

US President Joe Biden then said that Russian accusations that Kyiv has biological and chemical weapons are false and illustrate that Russian President Vladimir Putin is considering using them himself against Ukraine.