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Ukraine is calling for an increase in Western heavy weapons after Russia committed the bulk of its firepower to the eastern Donbas region, a topic expected to feature prominently at a meeting of NATO defence ministers on Wednesday in Brussels.
Ukraine says more than 500 civilians are trapped alongside soldiers inside Azot, a chemical factory where its forces have resisted weeks of Russian bombardment and assaults that have reduced much of Sievierodonetsk to ruins.
Fighters should "stop their senseless resistance and lay down arms" from 8am on Wednesday (Moscow time), Mikhail Mizintsev, head of Russia's National Defence Management Centre told the Interfax news agency.
Civilians would be let out through a humanitarian corridor, Mizintsev said.
The Azot bombardment echoes earlier battles over the Azovstal steelworks in the southern port of Mariupol, where hundreds of fighters and civilians took shelter from Russian shelling. Those inside surrendered in mid-May and were taken into Russian custody.
Shelling on Azot was so strong that "people can no longer stand it in the shelters, their psychological state is on edge," said regional governor Serhiy Gaidai of Luhansk, one of two eastern provinces Moscow claims on behalf of separatist proxies.
The battle for Luhansk's Sievierodonetsk - a city of barely more than 100,000 people before the war - is now the biggest fight in Ukraine as the conflict has become a war of attrition.
Ukraine is still trying to evacuate civilians after Russian forces destroyed the last bridge to the city. Ground has changed hands several times over the past few weeks, and Ukrainian officials have given little indication they will back down.
"We have to hold strong ... The more losses the enemy suffers, (the) less strength it will have to pursue its aggression," Zelenskiy said in an address late on Tuesday.
'UNABLE TO LEAVE'
Ukraine still holds Lysychansk, Sievierodonetsk's twin city on higher ground on the western bank of the Siverskyi Donets river. But with all the bridges now cut, its forces acknowledge a threat that they could be encircled.
Kyiv has said 100-200 of its soldiers are killed each day, with hundreds more wounded.
Russia gives no regular figures of its own losses but Western countries say they have been massive as President Vladimir Putin seeks to force Kyiv to cede full control of two provinces, Luhansk and Donetsk, collectively known as the Donbas.
Momentum in Sievierodonetsk has shifted several times over the past few weeks - with Russia concentrating its overwhelming artillery firepower on urban districts to obliterate resistance, then sending in ground troops vulnerable to counter-attacks.
Bigger battles could lie ahead for the wider Ukrainian-held pocket of the Donbas, nearly all on the opposite bank of the river which Russian forces have found difficult to cross. Ukraine says Russia plans to assault Sloviansk from the north and along a front near Bakhmut to the south.
In Donetsk province, critical infrastructure including homes, schools, hospitals and markets have been attacked over the past week, United Nations spokesman Stephane Dujarric told reporters in New York.
"This has made life nearly unbearable for people who are also facing severe water shortages, and at times are unable to leave their homes for days on end due to the fighting," Dujarric said.
Ukrainian officials have renewed pleas for the United States and its allies to send more and better artillery as well as tanks, drones and other heavy weapons.
Western countries have promised NATO-standard weapons - including advanced U.S. rockets. But deploying them is taking time, and Ukraine will require consistent Western support to transition to new supplies and systems as stocks dwindle of their Soviet-era weapons and munitions.
The meeting on Wednesday on the sidelines of a NATO defence ministerial is being led by US Defense Secretary Lloyd Austin. It is the third time the group of nearly 50 countries are meeting to discuss and coordinate Ukraine assistance.
Washington has committed about $4.6 billion in security assistance to Ukraine since Russia's February 24 invasion, including longer-range rocket systems, drones and advanced artillery.
But Zelenskiy said Ukraine does not have enough anti-missile systems to shoot down Russian projectiles targeting its cities.
"Our country does not have enough of them ... there can be no justification in delays in providing them," Zelenskiy said.