President Joe Biden has said the United States will send more advanced rocket systems to Ukraine, as Russian troops press their ferocious bid to complete the capture of a key eastern city.

The battle for Severodonetsk has grown in intensity in recent days, with heavy casualties on both the Ukrainian and Russian sides.

"The Russians are storming, consolidating in the centre of Severodonetsk, while continuing to destroy infrastructure and industrial facilities," Lugansk region governor Sergiy Gaiday wrote on Telegram early Wednesday.

One of the industrial hubs on Russia's path to taking the eastern Lugansk region, Severodonetsk has become a target of massive Russian firepower since the failed attempt to capture Kyiv.

The Russians now control most of the destroyed city, according to regional authorities.

But in a boost for the outgunned Ukrainian military, Biden has confirmed that more US weaponry is on the way.

"We will provide the Ukrainians with more advanced rocket systems and munitions that will enable them to more precisely strike key targets on the battlefield in Ukraine," Biden wrote in The New York Times.

A US official told reporters the weapons being sent are Himars, or the High Mobility Artillery Rocket System, which have precision-guided munitions and a longer range than those currently deployed by Ukraine.

The Himars are the centerpiece of a $700 million package being unveiled Wednesday that includes air surveillance radar, more Javelin short-range anti-tank rockets, artillery ammunition, helicopters, vehicles and spare parts, the official said.

The US is attempting to help Kyiv's war effort while not being seen as a direct belligerent, and the official stressed that while the weapons would be used to "repel Russian advances on Ukrainian territory", they would not be "used against Russia".

While some analysts have suggested the Himars could be a "game-changer", others say they should not be expected to suddenly turn the tables for Ukrainian forces struggling under Russian artillery fire.

'Just crazy'

The US announcement came shortly after Russian forces struck the nitric acid tank in Severodonetsk, prompting the local governor to warn people to stay indoors.

Ukrainian President Volodymyr Zelensky said that "given the presence of large-scale chemical production in Severodonetsk, the Russian army's strikes there, including blind air bombing, are just crazy".

Meanwhile, in Brussels, European Union leaders were split over banning natural gas from Moscow after agreeing to embargo two-thirds of its oil to tighten the economic screws.

Denmark on Wednesday was set to become the latest European country to be targeted by Russia over gas exports, following the Netherlands, Finland, Poland and Bulgaria.

Danish energy firm Orsted said Russian monopoly Gazprom Export would cut gas supplies after the Danes refused to pay in rubles, a demand Moscow is making of "unfriendly countries" in a bid to sidestep crippling Western sanctions.

Also on Wednesday, residents of Denmark will vote on whether to overturn the country's opt-out on the EU's common defence policy, a referendum that comes on the heels of neighbouring Finland's and Sweden's historic applications for NATO membership

The situation on the eastern frontline in Donbas, meanwhile, has become increasingly desperate, with Ukrainian towns facing near-constant shelling from Russian forces.

French journalist Frederic Leclerc-Imhoff was killed Monday while covering civilian evacuations in the Donbas.

A 'few thousand' war crimes

Ukraine's prosecutor general Iryna Venediktova said authorities had identified a "few thousand" cases of war crimes in the Donbas, including murder, torture and the forced displacement of children.

The key Zelensky aide, who met with international counterparts in The Hague on Tuesday, said Kyiv was already going to prosecute 80 suspects for alleged war crimes on Ukrainian soil.

A Ukrainian court on Tuesday jailed two Russian soldiers for 11 and a half years for shelling two villages in the northeastern Kharkiv region. Earlier this month, another was jailed for life for murdering a civilian.

Russia's invasion of its pro-Western neighbour is also threatening a global food crisis, with Ukraine's huge grain harvest effectively taken off the world market.

French President Emmanuel Macron said Tuesday that he and German Chancellor Olaf Scholz had urged Russian leader Vladimir Putin to end Russia's blockade of the Ukrainian port of Odessa.

But Russian Foreign Minister Sergei Lavrov said it was up to the West and Kyiv to resolve the crisis, starting with the lifting of sanctions.

In Kyiv, meanwhile, Ukrainian football fans were set to watch their national side play its first official match since Russia's invasion, facing Scotland in a World Cup qualifier later Wednesday in Glasgow.

"I am hoping for victory," 44-year-old army serviceman, Andriy Veres, told AFP.

"These days it is very important for the country, for all people, for all those who are fans and even for those who are not."