Russia was intensifying its attack on Ukraine's east on Tuesday as it seized a frontline city and sought a decisive victory in Mariupol, prompting Western governments to pledge more arms and sanctions.
Thousands of Russian troops backed by artillery and rocket barrages were advancing in what Ukrainian officials called the Battle of the Donbas.
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.
The biggest attack on a European state since 1945 has, however, seen nearly 5 million people flee abroad and reduced cities to rubble as the incursion drags on.
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. Reuters could not verify the details.
"The world watches the murder of children online and remains silent," adviser Mykhailo Podolyak wrote on Twitter.
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 renewed the proposal. Ukrainian commanders at Azovstal 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.
The United States, Canada, Britain, France and Germany pledged more support to Ukraine.
"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.
"It is unfair that Ukraine still has to ask for (weapons) which have been sitting for years in the storage depots of our partners" President Volodymyr Zelenskiy said in a video address.
U.N. Secretary-General Antonio Guterres called for a four-day humanitarian pause in the fighting this coming weekend, when Orthodox Christians celebrate Easter, to allow civilians to escape and humanitarian aid to be delivered.
Russia says it launched what it calls a "special military operation" on Feb. 24 to demilitarise and "denazify" Ukraine. Kyiv and its Western allies reject that as a false pretext.
