President Volodymyr Zelenskiy said Moscow's massive air and artillery attacks were aimed at destroying the entire Donbas region and urged Ukraine's allies to accelerate the shipment of heavy weapons to match Russia on the battlefield.

On the diplomatic front, European leaders on Thursday will formally set Ukraine on the long road to EU membership at a summit in Brussels. Though mainly symbolic, the move will help lift national morale after four-months of bloody conflict that has killed thousands, displaced millions and destroyed cities.

"We must free our land and achieve victory, but more quickly, a lot more quickly," Zelenskiy said in a video address released early Thursday, reiterating Ukrainian demands for larger and faster weapons.

"There were massive air and artillery strikes in Donbas. The occupier's goal here is unchanged, they want to destroy the entire Donbas step-by-step," he said.

"This is why we again and again emphasize the acceleration of arm deliveries to Ukraine. What is quickly needed is parity on the battlefield in order to halt this diabolical armada and push it beyond Ukraine's borders."

In an indication that the battle for the Donbas was becoming more difficult, Tass news agency cited pro-Moscow separatists as saying they had captured most of Vovchoyarivka, a village some 12 km (7.46 miles) south west of the city of Lysychansk. If true, Lysychansk would be at greater risk of being cut off.

Reuters was unable to immediately confirm the report.

As the fierce war of attrition grinds on in the Donbas, Russian forces resumed pounding Ukraine's second-biggest city Kharkiv near the Russian border.

The Russian strikes on Tuesday and Wednesday on Kharkiv were the worst for weeks in an area where normal life had been returning since Ukraine pushed Moscow's forces back last month.

Kyiv characterised the strikes, which reportedly killed at least 20 people, as a bid to force Ukraine to pull resources from the main battlefields in the Donbas to protect civilians.

"There is no letup in the shelling of civilians by the Russian occupiers," Oleh Synehubov, governor of Kharkiv region, wrote on the Telegram messaging app. "This is evidence that we cannot expect the same scenario as in Chernihiv or Kyiv, with Russian forces withdrawing under pressure."

EU MEMBERSHIP

Russia invaded Ukraine on Feb. 24 in what the Kremlin calls a "special operation" to ensure Russian security and denazify Ukraine. Ukraine and the West call this a baseless pretext for a war of choice that has raised fears of wider conflict in Europe.

Russia has long opposed closer links between Ukraine, a former Soviet state, and Western clubs like the European Union (EU) and the NATO military alliance.

Zelenskiy said he had spoken to 11 EU leaders on Wednesday about Ukraine's candidacy and will make more calls on Thursday. He said earlier he believed all 27 EU countries would support Ukraine's candidate status.

"We deserve it," Zelenskiy told crowds in Amsterdam via video link.

Diplomats say it will take Ukraine a decade or more to meet the criteria for joining the EU. But EU leaders say the bloc must make a gesture that recognises Ukraine's sacrifice.

The war in Ukraine has had a massive impact on the global economy and European security, driving up gas, oil and food prices, pushing the EU to reduce its heavy reliance on Russian energy and prompting Finland and Sweden to seek NATO membership.

The EU will temporarily shift back to coal to cope with dwindling Russian gas flows without derailing longer-term climate goals, an EU official said on Wednesday, as a tight gas market and soaring prices set off a race for alternative fuels.

BATTLE FOR SIEVIERODONETSK

The battle for the frontline eastern city of Sievierodonetsk, where hundreds of civilians are trapped in a chemical plant, seesawed with Ukraine and Russia disputing who controlled the city.

Luhansk's regional governor, Serhiy Gaidai, said in an online post on Wednesday evening that Russian forces continue to build up reserves in Sievierodonetsk in an attempt to encircle Ukrainian troops.

He rejected Russian claims that its military already controls the city. "Battles are continuing," he told Ukrainian Television. "Russian forces do not have full control."

Moscow says Ukrainian forces in Sievierodonetsk, scene of the heaviest recent fighting, are trapped. It ordered them last week to surrender or die after the last bridge over the Siverskyi Donets river was destroyed.

Inside Russia, a fire tore through an oil refinery just 8 km (5 miles) from the frontier with Donbas territory controlled by pro-Russian separatists, after what the refinery described as a cross-border attack on Wednesday by two drones.

There was no immediate Ukrainian comment on the strike, which suspended production at the Novoshakhtinsk refinery.

Ukraine generally does not comment on reports of attacks on Russian infrastructure near the border, which it has called "karma" for Russian attacks on Ukraine.