Russia said it would establish a humanitarian corridor to evacuate civilians from a chemical plant in Severodonetsk starting Wednesday as Ukrainian forces wage a desperate battle for control of the city.

The industrial hub is under intense bombardment as Russia focuses its offensive on the eastern Donbas region in an effort to seize a swathe of Ukraine.

Moscow's forces have intensified efforts to cut off Ukrainian troops remaining in the city, destroying all three bridges which connect it across a river to the twin city of Lysychansk.

NATO chief Jens Stoltenberg meanwhile urged allies to send more heavy weapons to Ukraine, and said officials from the alliance would be discussing the subject at talks Wednesday.

About 500 civilians are taking shelter in Severodonetsk's Azot chemical plant, according to the head of the city's administration.

The Russian defence ministry announced a humanitarian corridor would be established on Wednesday for evacuations from the plant, saying it was "guided by the principles of humanity".

Evacuees would be transported to the city of Svatovo in the separatist-held region of Lugansk, Moscow said, urging those holding out at the plant to cease their "senseless resistance".

There was no response from Kyiv to the announcement, and in a video address Tuesday evening, Ukrainian President Volodymyr Zelensky lamented "painful losses" in the ongoing fighting.

"But we must stay strong. This is our nation... Hanging in there in Donbas is crucial. Donbas is the key to deciding who will dominate in the coming weeks."

Following its February invasion, Russia was repelled from Kyiv and other parts of Ukraine, prompting it to focus its offensive on Donbas, a mainly Russian-speaking region partly held by pro-Kremlin separatists since 2014.

Capturing Severodonetsk has become a key goal, as it would open the road to Sloviansk and another major city, Kramatorsk.

NATO urges heavy weapons

Speaking in The Hague, NATO chief Stoltenberg urged Western countries to send the Ukrainians more heavy armaments, as they "absolutely depend on that to be able to stand up against the brutal Russian invasion".

Addressing a press conference after meeting the leaders of seven European NATO allies, he added that NATO officials would discuss coordinating further support including heavy weaponry at a meeting in Brussels Wednesday.

Zelensky meanwhile told reporters that he regretted what he called "the restrained behavior of some leaders" which, he said, had "slowed down arms supplies very much".

Ukraine has only received 10 percent of the arms it had requested from the West, Kyiv's deputy defense minister said.

Kyiv's forces face an increasingly desperate situation in Severodonetsk, with Ukrainian authorities estimating the Russians now control up to 80 percent of the city as they seek to encircle it.

From an elevated position in Lysychansk, an AFP team saw black smoke rising from the Azot factory in Severodonetsk and another area in the city.

The Ukrainian military is using the high ground to exchange fire with Russian forces fighting for control of Severodonetsk, just across the water.

Lysychansk pensioner Valentina sat on the porch of her ground floor apartment, where she lives alone, her two walking sticks to hand.

"It's scary, very scary," said the 83-year-old former farm worker.

"Why can't they agree at last, for God's sake, just shake hands?"

Along the road from Lysychansk to Kramatorsk, Ukrainian forces were transporting more weapons systems to the front, while specialist vehicles carried tanks for repair.

In the town of Novodruzhesk, close to Lysychansk, there was still a smell of burning and smoke from houses that had been destroyed by fire from shelling at the weekend.

"It's not safe anywhere, it just depends on the time of day, that's all," said a soldier standing at a fire station with a skull logo on his sleeve.

Britons blacklisted

As tensions soar with the West, Russia announced it was blacklisting 49 British citizens, including defense officials and prominent reporters and editors from the BBC, The Financial Times and The Guardian.

The Russian foreign ministry said that the journalists targeted were "involved in the deliberate dissemination of false and one-sided information".

In New York, a senior UN official warned Tuesday that Ukrainian children should not be adopted in Russia, where several thousand young people are believed to have been moved since Moscow's February invasion.

"We're reiterating, including to the Russian Federation, that adoption should never occur during or immediately after emergencies," Asfhan Khan, the UN Children's Fund (UNICEF) regional director for Europe and Central Asia, told reporters.

Such children cannot be assumed to be orphans and their movement must be voluntary, Khan added.

The Kremlin, meanwhile, said it had not received a request from London to intervene in the case of two Britons sentenced to death by pro-Moscow separatist authorities in eastern Ukraine.

Aiden Aslin and Shaun Pinner, along with Moroccan Brahim Saadun, were convicted of acting as mercenaries for Ukraine by the self-proclaimed Donetsk People's Republic.