Bolstered by support from allies, Ukraine vowed on Saturday to prevail against Moscow as it battled Russian assaults near a key eastern city and multiple locations came under shell and missile attack.

European Union countries are expected at a summit next week to grant Ukraine EU candidate status following a recommendation from the bloc's executive on Friday. That put Kyiv on course to realise an aspiration that was seen as out of reach before Russia's military campaign began in February, even if actual membership could take years.

Meanwhile, British Prime Minister Boris Johnson made a surprise visit to Kyiv on Friday and offered training for Ukrainian forces. Back in Britain on Saturday, he stressed the need to avoid "Ukraine fatigue" after nearly four months of war.

"The Russians are grinding forward inch by inch and it is vital for us to show what we know to be true, which is that Ukraine can win and will win," he told reporters.

"When Ukraine fatigue is setting in, it is very important to show that we are with them for the long haul and we are giving them the strategic resilience that they need."

On the battlefields on Saturday, Sievierodonetsk, a prime target in Moscow's offensive to seize full control of the eastern region of Luhansk, was again under heavy artillery and rocket fire as the Russian forces attacked areas outside the industrial city, the Ukrainian military said.

Rodion Miroshnik, of the self-styled separatist administration of the Luhansk People's Republic, posted a video on Telegram of what he said was a large orange-coloured cloud, adding that he could not tell if the blast had occurred in or near the city.

Luhansk Governor Serhiy Gaidai said Russian attacks on towns just south of Sievierodonetsk were repulsed by Ukrainian forces, although the situation in satellite villages was "difficult."

"The Russians have thrown all their reserves in the direction of Sievierodonetsk and Bakhmut," he said in an online post. "They are trying to establish full control over the regional centre and to cut the Lysychansk-Bakhmut highway. They're having no success, they are dying en masse."

Gaidai said the city of Lysychansk was under constant shelling but remained fully in Ukrainian hands, although a "quiet" evacuation was underway, and humanitarian convoys were being brought in daily. He said a key highway out of the city was now impassable due to Russian shelling.

To the northwest, several Russian missiles hit a gasworks in Izium district, and Russian rockets rained on a suburb of Kharkiv, Ukraine's second-largest city, hitting a municipal building and starting a fire in a block of flats, but causing no casualties, Ukrainian authorities said. (Reuters)