Russian officials on Wednesday told Ukrainian fighters still holding out in Mariupol to give up as it pushed for a decisive victory in its new eastern offensive.

It is reported that thousands of Russian troops backed by artillery and rocket barrages were advancing in what Ukrainian officials called the Battle of the Donbas.

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Russia’s nearly eight-week-long invasion has taken longer than many expected while still failing to capture any of the biggest cities, forcing Moscow to refocus in and around separatist regions.

However, the biggest attack on a European state since 1945 has seen nearly 5 million people flee abroad and reduced cities to rubble as the incursion drags on.

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Reuters reported that Russia was hitting the Azovstal steel plant, the main remaining stronghold in Mariupol, with bunker-buster bombs, a Ukrainian presidential adviser said late on Tuesday.

“The world watches the murder of children online and remains silent,” adviser Mykhailo Podolyak wrote on Twitter.

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After an earlier ultimatum to surrender lapsed and as midnight approached, Russia’s defence ministry said not a single Ukrainian soldier had laid down their weapons and it renewed the proposal. Ukrainian commanders have vowed not to surrender.

“Russia’s armed forces, based purely on humanitarian principles, again propose that the fighters of nationalist battalions and foreign mercenaries cease their military operations from 1400 Moscow time on 20th April and lay down arms,” the Russian Defence Ministry said.

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The United States, Canada and Britain said they would send more artillery weaponry.

“We will continue to provide them more ammunition, as we will provide them more military assistance,” White House spokesperson Jen Psaki said, adding that new sanctions were being prepared.

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Russia says it launched what it calls a “special military operation” on February 24 to demilitarise and “denazify” Ukraine. Kyiv and its Western allies reject that as a false pretext.