Meanwhile, in Kharkiv, the Ukrainian military says they have found 103 bodies, believed to be of Russian troops, in territories liberated from the Kremlin's forces.
Russia's war in Ukraine could take years, NATO Secretary-General Jens Stoltenberg told a German weekly newspaper, asking the invaded country's western allies to continue supporting it.
"We must prepare for the fact that it could take years. We must not let up in supporting Ukraine," Stoltenberg told Bild am Sonntag. "Even if the costs are high, not only for military support, also because of rising energy and food prices."
Stoltenberg stressed, though, that "the costs of food and fuel are nothing compared with those paid daily by the Ukrainians on the front line."
If Russian President Vladimir Putin should reach his objectives in Ukraine, like when he annexed Crimea in 2014, "we would have to pay an even greater price," Stoltenberg added.
British Prime Minister Boris Johnson has expressed concerns "that a bit of Ukraine fatigue is starting to set in around the world" and has urged support for Ukrainian efforts to try to roll back the Russian invasion.
"It would be a catastrophe if Putin won. He'd love nothing more than to say, 'Let's freeze this conflict, let's have a cease-fire,'" Johnson said on Saturday, a day after a surprise visit to Kyiv, where he met with Ukrainian President Volodymyr Zelenskyy and offered offer continued aid and military training.
Troop morale suffers on both sides
Four months of brutal fighting in Ukraine appear to be straining the morale of troops on both sides, prompting desertions and rebellion against officers' orders, British defence officials said Sunday.
"Combat units from both sides are committed to intense combat in the Donbas and are likely experiencing variable morale," Britain's defence ministry said in its daily assessment of the war.
"Ukrainian forces have likely suffered desertions in recent weeks," the assessment said, but added that "Russian morale highly likely remains especially troubled."
"Cases of whole Russian units refusing orders and armed stand-offs between officers and their troops continue to occur," the assessment said.
Separately, the Ukrainian Main Intelligence Directorate released what it said were intercepted phone calls in which Russian soldiers complained about frontline conditions, poor equipment, and overall lack of personnel, according to a report by the Institute for the Study of War.
Britain's defence ministry said that both Russia and Ukraine have continued to conduct heavy artillery bombardments on axes to the north, east and south of the Sieverodonetsk pocket, but with little change in the front line.
"It is a very difficult situation in Sievierodonetsk, where the enemy in the middle of the city is conducting round-the-clock aerial reconnaissance with drones, adjusting fire, quickly adjusting to our changes," Luhansk governor Serhiy Haidai said via Telegram on Sunday.
Russian troops continue attacks in Ukraine's south
On Saturday, Zelenskyy made a trip south from Kyiv to visit troops and hospital workers in the Mykolaiv and Odesa regions along the Black Sea. He handed out awards to dozens of people at every stop, shaking their hands and thanking them for their service.
Sometime after Zelenskyy left Mykolaiv, "the enemy carried out fire damage against units of the Defence Forces with cannon and rocket artillery in the areas of the settlements of Pravdyne, Posad-Pokrovskoe and Blahodatne," according to the Ukrainian army briefing on Sunday.
In other attacks in the south, two people were killed in the shelling of the Galitsyn community in the Mykolaiv region and the shelling of the Bashtansky district is continuing, Ukraine's military operational command for the area said on Sunday.
Russia's defence ministry said seaborne missiles destroyed a plant in Mykolaiv city where Western-supplied howitzers and armoured vehicles were stored.
Western-supplied heavy weapons are reaching the front lines. But Ukraine's leaders have insisted for weeks that they need more arms and they need them sooner.
On Sunday Pope Francis, despite having lamented arms build-ups, added his own admonition to those who might lose focus on Ukraine, which he has said deserves to defend itself.
"And let's not forget the martyred Ukrainian people at this moment," Francis told the public in St. Peter's Square. He prodded them to ask themselves, "'What did I do today for the Ukrainian people?"