Starting on the long path to EU membership will be a huge boost to morale in the embattled country, as Russian assaults on two cities in the eastern Donbas region move toward a "fearsome climax", according to a Ukrainian government adviser.

"Ukraine's future is in the EU," President Volodymyr Zelenskiy wrote on Twitter after the official announcement.

"A historic moment," European Council chief Charles Michel tweeted, adding: "Our future is together."

The approval of the Kyiv government's application by EU leaders meeting in Brussels will anger Russia as it struggles to impose its will on Ukraine. Moldova also became an official candidate on Thursday, signalling the bloc's intention to reach deep into the former Soviet Union.

Friday will mark four months since Russian President Vladimir Putin sent troops across the border in what he calls a "special military operation" sparked in part by Western encroachment into what Russia considers its sphere of influence.

Russia has focused its campaign on southern and eastern Ukraine after its advance on the capital in the early stages of the conflict was thwarted by Ukrainian resistance.

The war of attrition in the Donbas - Ukraine's industrial heartland - is most critical in the twin cities of Sievierodonetsk and Lysychansk, which sit on opposite banks of the Siverskyi Donets River in Luhansk province.

The battle there is "entering a sort of fearsome climax", said Oleksiy Arestovych, an adviser to Zelenskiy.

Hot summer

Russian forces were trying to encircle Ukrainian troops defending Lysychansk, senior Ukrainian defence official Oleksiy Gromov said in a briefing on Thursday.

Luhansk governor Serhiy Gaidai said separately that all Lysychansk was within reach of Russian fire and that Ukrainian troops there might retreat to new positions to avoid being trapped.

Russian-backed separatist forces said there was fierce fighting underway around Ukrainian positions in Hirske, which lies on the western side of the main north-south road to Lysychansk, and Zolote, another settlement to the south.

Ukrainian forces were defending Sievierodonetsk and nearby Zolote and Vovchoyrovka, Gaidai said, but Russian troops had captured Loskutivka and Rai-Oleksandrivka to the south. Hundreds of civilians are trapped in a chemical plant in Sievierodonetsk.

On the southern front, Russian forces struck Ukrainian army fuel tanks and military equipment near Mykolaiv with high-precision weapons, Russia's defence ministry said, quoted by the Interfax news agency.

A river port and ship-building centre just off the Black Sea, Mykolaiv has been a bastion against Russian efforts to push West towards Ukraine's main port city of Odesa.

Zelenskiy urged Ukraine's allies to speed up shipments of heavy weapons to match Russia on the battlefield. "We must free our land and achieve victory, but more quickly, a lot more quickly," he said in a video address early on Thursday.

Later, Ukrainian defence minister said HIMARS multiple rocket systems had arrived from the United States. With a range of 70 km (44 miles), the systems can challenge the Russian artillery batteries that have bludgeoned Ukrainian cities from afar.

The United States will provide an additional $450 million in security assistance to Ukraine, including more long-range rocket systems, U.S. officials said on Thursday.

Shield for the EU

Russia has long opposed closer links between Ukraine, a fellow former Soviet republic, and Western groupings like the European Union and the NATO military alliance.

Diplomats say it will take Ukraine a decade or more to meet the criteria for joining the EU.

But European Commission President Ursula von der Leyen said she was convinced that Ukraine and Moldova will move as swiftly as possible to implement necessary reforms.

Their move to join the EU runs alongside applications by Sweden and Finland to enter NATO in the wake of the Russian invasion - indications that the Kremlin's military actions have backfired on its geopolitical aims.

In Kyiv, where mass protests eight years ago ousted the then-president after he broke a promise to develop closer ties with the EU, 22-year-old serviceman Volodymyr Yanishan welcomed Ukraine's candidate status.

"It means that people almost reached what we have been striving for since 2014, in a bloody fight which cost us much effort... I think the majority will be glad and it means changes for better."