Russia gave Ukrainian fighters still holding out in Mariupol a fresh ultimatum to surrender on Wednesday as it pushed for a decisive victory in its new eastern offensive, while Western governments pledged more military help to Kyiv.
Thousands of Russian troops backed by artillery and rocket barrages were advancing in what Ukrainian officials have called the Battle of the Donbas.
Russia's nearly eight-week-long invasion has failed to capture any of Ukraine's largest 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.
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 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.
The United States, Canada and Britain said they would send more artillery weaponry, and the White House said new sanctions were being prepared.
US President Joe Biden is expected to announce a new military aid package about the same size as last week's $800 million one in the coming days, sources told Reuters.
UN 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's war in Ukraine is to blame for exacerbating "already dire" world food insecurity, with price and supply shocks adding to global inflationary pressures, US Treasury Secretary Janet Yellen said.
Russia says it launched what it calls a "special military operation" on February 24 to demilitarise Ukraine. Kyiv and its Western allies reject that as a false pretext.
Ukraine said the new assault had resulted in the capture of Kreminna, an administrative centre of 18,000 people in Luhansk, one of the two Donbas provinces.
Russian Foreign Minister Sergei Lavrov confirmed that "another stage of this operation is beginning".
Driven back by Ukrainian forces in March from an assault on Kyiv in the north, Russia has instead poured troops into the east for the Donbas offensive.
It has also made long-distance strikes at other targets including the capital and the northeastern city of Kharkiv, where at least four people were killed by missiles, authorities said on Tuesday.
In one suburban street, the body of an elderly man lay face down near a park, a thick ribbon of blood running into the gutter.
"He worked in security not far from here," a resident named Maksym told Reuters. "The shelling began and everyone fled. Then we came out here, the old guy was already dead."