Ukraine says more than 500 civilians are trapped alongside soldiers inside the Azot chemical factory.
Ukraine showed no signs of obeying a Russian ultimatum to surrender the eastern city of Severodonetsk early on Wednesday as NATO defence ministers gathered in Brussels to discuss sending more heavy weapons to replenish Kyiv's dwindling stocks.
Russia had told Ukrainian forces holed up in a chemical plant in the shattered city to stop "senseless resistance and lay down arms" from Wednesday morning, pressing its advantage in the battle for control of eastern Ukraine.
Civilians would be let out through a humanitarian corridor, Mikhail Mizintsev, head of Russia's National Defence Management Centre told the Interfax news agency.
Ukraine says more than 500 civilians are trapped alongside soldiers inside the Azot chemical factory where its forces have resisted weeks of Russian bombardment and assaults that have reduced much of Severodonetsk to ruins.
"It's getting harder, but our military are holding back the enemy from three directions at once," Serhiy Gaidai, governor of the Luhansk region containing Severodonetsk, posted online just before Russia's 8 a.m. Moscow time (0500 GMT) deadline.
"They're defending Sevierodonetsk and not letting them to advance to Lysychansk," he said, referring to the twin city held by Ukraine on the opposite bank of the Siverskyi Donets river.
"Nevertheless, the Russians are close and the population is suffering and homes are being destroyed."
Luhansk is one of two eastern provinces Moscow claims on behalf of separatist proxies. Together they make up the Donbas, an industrial Ukrainian region where Russia has focused its assault after failing to take Ukraine's capital Kyiv in March.
The Ukrainian general staff said in the early hours that its soldiers were still repelling Russian assaults on the city.
British intelligence said the fighters in the chemical plant could survive underground, and that Russian forces would likely remain focused on the plant, preventing them turning their firepower elsewhere.
Reuters could not immediately verify the battlefield accounts, or what happened after the ultimatum passed.
Echoes of Mariupol
The Azot bombardment echoes the earlier siege of 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.
The Russian assault on Severodonetsk - a city of barely more than 100,000 people before the war - is the current focal point of what has been called the battle of the Donbas.
Kyiv has said 100-200 of its soldiers are killed each day, with hundreds more wounded in some of the bloodiest fighting since Russia's Feb. 24 invasion.
Ukraine said on Tuesday it was still trying to evacuate civilians after Russian forces destroyed the last bridge linking Severodonetsk with Lysychansk, which lies on higher ground on the western bank of the Siverskyi Donets river.
With all the bridges leading from Severodonetsk now destroyed, Ukrainian forces risk being encircled.
"We have to hold strong ... The more losses the enemy suffers, (the) less strength it will have to pursue its aggression," Zelensky said in an address late Tuesday.
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 US rockets. But deploying them is taking time, and Ukraine will require consistent Western support to transition to new supplies and weapons systems as stocks dwindle of their Soviet-era weapons and munitions.
The meeting in Brussels on Wednesday on the sidelines of a NATO defence ministerial is being led by U.S. Defense Secretary Lloyd Austin. It is the third time the group of nearly 50 countries are meeting to discuss and coordinate assistance to Ukraine.
In May, the U.S. Senate passed a bill to provide $40 billion in additional aid to Ukraine, including $15 billion for defense measures, and has promised longer-range rocket systems, drones and advanced artillery.
But Zelensky said Ukraine does not have enough anti-missile systems to protect its cities, adding that "there can be no justification in delays in providing them."
'Unable To Leave'
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 the Donbas and a swathe of southern Ukraine. Putin calls the war a special military operation.
Momentum in Severodonetsk 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.
Elsewhere in the Donbas, 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.
To the south, Ukraine's military said it had conducted three air strikes against troop concentrations, fuel depots and military equipment in the Kherson region.
While Western sanctions have hit Russia's economy hard, resulting global shortages of oil and grain have sent energy and commodity prices soaring. A speech that Putin is set to deliver on Friday at the St Petersburg International Economic Forum will be closely watched
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