EU leaders met in Brussels on Thursday to discuss making Ukraine a candidate to join the bloc, a "decisive" moment likely to infuriate Russia as its forces battled stiff resistance to advance in the embattled eastern Donbas region.

Western officials also denounced Moscow's "weaponising" of its key gas and grain exports, with a US official warning of further retaliation measures at a G7 summit in Germany starting Sunday.

Germany ratcheted up an emergency gas plan to its second alert level, just one short of the maximum that could require rationing in Europe's largest economy after Russia slashed its supplies.

"Gas is now a scarce commodity," Economy Minister Robert Habeck told reporters, urging households to cut back on use.

Demonstrators gather outside the European Headquarters part as they protest in support of Ukraine's application for EU candidacy status during an EU-Western Balkans leaders' meeting in Brussels on June 23, 2022. (Photo by Kenzo TRIBOUILLARD / AFP)

In Ankara meanwhile, British Foreign Secretary Liz Truss accused Russian President Vladimir Putin of "weaponising hunger" by preventing grain shipments from leaving Ukraine ports, raising the spectre of shortages worldwide.

"We are very clear that this grain crisis is urgent, that it needs to be solved within the next month. Otherwise we could see devastating consequences," Truss said after talks with her Turkish counterpart Mevlut Cavusoglu.

Moscow and Ankara have negotiated for weeks on getting millions of tonnes of desperately needed grain out of the war zone and on to Africa and the Middle East, so far to no avail.

The potential consequences for Ukraine's allies loomed large over the country's EU candidate status talks in Brussels, and the G7 and NATO meetings in the following days.

Ukrainian President Volodymyr Zelensky said he had conducted a "telephone marathon" ahead of the meeting, making his case to 11 European leaders on Wednesday alone.

"This is a decisive moment for the European Union, this is also a geopolitical choice that we will make today," EU council president Charles Michel told journalists ahead of the summit.

Russian gains

While the European Commission-backed candidacy is widely expected to be approved, some members have been lukewarm about Ukraine's status, and any accession process is likely to take years if not decades.

On the ground in the Donbas, the situation was becoming increasingly urgent as Russian forces tightened their grip on the strategically important city of Severodonetsk, as well as its twin city of Lysychansk across the Donets river.

Taking the two cities would give Moscow control of the whole of Lugansk, allowing Russia to press further into Donbas.

Britain's defence ministry said some Ukrainian units had probably withdrawn "to avoid being encircled" as Russian troops advanced slowly but steadily toward Lysychansk.

"Russia's improved performance in this sector is likely a result of recent unit reinforcement and heavy concentration of fire," it said in its latest intelligence update.

"The Russian army is... just destroying everything" in Lysychansk, said Sergiy Gaiday, governor of the Lugansk region.

"It's just hell out there," after four months of shelling in Severodonetsk, he wrote later, vowing that "Our boys are holding their positions and will continue to hold on as long as necessary."

'Only grannies left'

After being pushed back from Kyiv and other parts of Ukraine in the initial weeks of the invasion launched on February 24, Moscow is seeking to seize a vast eastern swathe of the country.

But daily bombardments also continue elsewhere.

The northeastern city of Kharkiv near the Russian border was near empty on Wednesday, AFP reporters said, a day after shelling by Moscow's forces killed five people there.

"Last night the building next to mine collapsed from the bombardment while I was sleeping," said Leyla Shoydhry, a young woman in a park near the opera house.

Roman Pohuliay, a 19-year-old in a pink sweatshirt, said most residents had fled the city.

"Only the grannies are left," he said.

Zelensky again pressed allies Wednesday for the rapid supply of more arms, having earlier accused the Russian army of "brutal and cynical" shelling in the eastern Kharkiv region, where the governor said 15 people had been killed in a day.

In the central city of Zaporizhzhia, meanwhile, women were training to use Kalashnikov assault rifles in urban combat as Russian forces edged nearer.

"When you can do something, it's not so scary to take a machine gun in your hands," said Ulyana Kiyashko, 29, after moving through an improvised combat zone in a basement.

Lithuania in cross-hairs

Away from the battlefield, Moscow this week summoned Brussels' ambassador in a dispute with EU member Lithuania over the country's restrictions on rail traffic to the Russian outpost of Kaliningrad.

The coastal territory, annexed from Germany after World War II, is about 1,000 miles (1,600 kilometres) from Moscow, and borders Lithuania and Poland but has no land border with Russia.

By blocking goods arriving from Russia, Lithuania says it is simply adhering to European Union-wide sanctions on Moscow.

The United States made clear its commitment to Lithuania as a NATO ally, while Germany urged Russia not to "violate international law" by retaliating.