Incessant shelling has left Ukrainian forces defending ruins in Sievierodonetsk, but their refusal to withdraw is stalling a massive Russian offensive across the Donbas.

"Some 90 per cent of buildings are damaged. More than two-thirds of the city's housing stock has been completely destroyed. There is no telecommunication," Ukrainian President Volodymyr Zelenskiy said in a televised speech on Monday.

"Capturing Sievierodonetsk is a fundamental task for the occupiers ... We do all we can to hold this advance."

Zelenskiy visited Ukrainian troops on the frontlines in northeastern Kharkiv region on Sunday, his first trip outside the Kyiv region since the invasion began.

"You risk your lives for us all and for our country," the president's office quoted him as telling soldiers as he handed out commendations and gifts.

Russian Foreign Minister Sergei Lavrov said the "liberation" of the Donbas, an industrial region which includes Luhansk and Donetsk, was an "unconditional priority" for Moscow.

Ukrainian forces in the Donbas said they were on the defensive all day on Sunday. Russian forces fired on 46 communities in the Donetsk and Luhansk regions, killing at least three civilians, wounding two others or destroying or damaging 62 civilian buildings.

Russian shelling also continued on Sunday across several regions such as in Novy Buh in Mykolaiv and Sumy.

A Ukrainian soldier on patrol in trenches near the town of Bakhmut, southwest of Sievierodonetsk, spoke of a nagging fear that his government could be drawn into negotiating an end to the conflict that would result in Ukraine losing territory.

"You know now what I'm most afraid of, now that the fighting is so intense, so tough?" Dmytro, a former English language teacher, told Reuters television.

"That we would be told: That's it, stop it, we have a ceasefire."

"A negotiated settlement can only happen on Ukrainian terms and at present if it happened it would be horror," he said, adding that such a move could end Zelenskiy's career.

German Economy Minister Robert Habeck expressed fears that EU unity was "starting to crumble" as the bloc's leaders are set to meet on Monday and Tuesday in a summit to discuss a new sanctions package against Russia including the oil embargo.

Draft conclusions of the meeting, seen by Reuters, showed that while the EU will be generous with verbal support for Kyiv, there will be little progress on any of the main issues.

EU governments have been unable to agree on a sixth package of sanctions against Moscow because a proposed embargo on Russian oil is not acceptable to Hungary and a big problem for Slovakia and the Czech Republic.

Having failed to take the capital Kyiv in the early phase of the war, Russia is seeking to consolidate its grip on the Donbas, large parts of which are already controlled by Moscow-backed separatists.

The Ukrainian government urged the West to provide more longer-range weapons to turn the tide in the war, now in its fourth month. Zelenskiy said he expected "good news" in the coming days.

US officials have said such weapons systems are actively being considered. Canada has asked South Korea to supply it with artillery rounds, Seoul said on Monday, apparently to "backfill" supplies that Ottawa has sent to Ukraine.

Analysts at the Institute for the Study of War in Washington said the Russians had still not managed to encircle Sievierodonetsk and the Ukrainian defenders had inflicted "fearful casualties" on them.

The Ukrainians were taking serious losses themselves, civilians as well as combatants, they said in a briefing paper.

Russia's focus on Sievierodonetsk had drawn resources from other battlefronts and as result they had made little progress elsewhere, the analysts said.

Australian Associated Press