Ukraine on Wednesday showed no signs of obeying a Russian ultimatum to surrender the eastern city of Severodonetsk as NATO defense 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.

Plans announced by Moscow to open a humanitarian corridor for civilians taking shelter in the plant were disrupted, Russian-backed separatists said, blaming shelling by Ukraine.

The Mayor of Severodonetsk Oleksandr Stryuk said that after the early morning deadline passed Russian forces were trying to storm the city from several directions but claimed Ukrainian forces continued to defend it and were not completely cut off.

"We are trying to push the enemy toward the city center," he said on television, without referring to the ultimatum. "This is an ongoing situation with partial successes and tactical retreats," he added.

"The escape routes are dangerous, but there are some," he said.

His comments echoed those by Serhiy Gaidai, governor of the Luhansk region containing Severodonetsk, posted online just before Russia's 8 a.m. Moscow time (5 a.m. GMT) deadline.

"It's getting harder, but our military are holding back the enemy from three directions at once," Gaidai posted online just before the ultimatum deadline.

"They're defending Severodonetsk and not letting them to advance to Lysychansk," he said, referring to the twin city held by Ukraine on the opposite bank of the Severskyi Donets river.

"Nevertheless, the Russians are close and the population is suffering and homes are being destroyed," he conceded.

Luhansk is one of two eastern provinces Moscow claims on behalf of separatist proxies. Together, they make up the Donbass, 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 Donbass.

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 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 Severskyi 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," Zelenskyy said in an address late Tuesday.

'Brussels, we are waiting'

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 weapons systems as stocks dwindle of their Soviet-era weapons and munitions.

The meeting in Brussels on Wednesday on the sidelines of a NATO defense 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 Zelenskyy said Ukraine does not have enough anti-missile systems, adding that "there can be no justification in delays."

His adviser, Mykhailo Podolyak, for his part, said the defenders of Severodonetsk wanted to know when the weapons would arrive. "Brussels, we are waiting for a decision," he wrote on Twitter.

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 Donbass 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 counterattacks.

Elsewhere in the Donbass, Ukraine says Russia plans to assault Sloviansk from the north and along a front near Bakhmut to the south.

To the south, Ukraine's military claimed it had conducted three airstrikes against troop concentrations, fuel depots and military equipment in the Kherson region.