LYSYCHANSK, Ukraine — Russian forces appeared poised to tighten the noose around thousands of Ukrainian troops near two strategically important cities in the fiercely contested Donbas region of eastern Ukraine on Sunday, mounting an assault on Ukrainian front lines that forced Ukraine to rush reinforcements to the area.
On a day of fighting that put even territory thought to be securely in Ukrainian hands in play, Prime Minister Boris Johnson of Britain and the NATO secretary general, Jens Stoltenberg, warned that the war could grind on for years. They urged Ukraine’s Western allies to settle in for the long haul as Russia moved aggressively to wear Ukraine down through what Mr. Johnson, writing in The Sunday Times of London, called a “campaign of attrition.”
The Russians made an initial breakthrough Sunday in Toshkivka, a small town southeast of the metropolitan area of Sievierodonetsk and Lysychansk, where vicious street-by-street fighting and artillery duels have raged for weeks. Sergei Haidai, the regional military governor, acknowledged that the Russians had “had success” in the Toshkivka area but said the occupiers “suffered defeat” after Ukrainian artillery went to Toshkivka’s defense.
It was not clear who controlled Toshkivka by nightfall Sunday. If Moscow’s forces succeed in eventually surrounding Sievierodonetsk and Lysychansk, however, thousands of Ukrainian fighters defending the two cities could be stranded. That would deliver a hard-won military victory to the Kremlin and bring Russian forces a step closer to fulfilling President Vladimir V. Putin’s objective of seizing all of Ukraine’s eastern Donbas region.
“The heavy fights in Sievierodonetsk are continuing,” Mr. Haidai said, adding that the Russians “fail and suffer massive losses.”
Phone communications in the area are limited, and the bridges leading into Sievierodonetsk have been destroyed, making it difficult to get an accurate picture of the situation there. The Institute for the Study of War, a Washington research group, said that Russia had made “marginal gains” around the city but that its offensive across the rest of Donbas had “largely stalled.”
“Russian forces will likely be able to seize Sievierodonetsk in the coming weeks, but at the cost of concentrating most of their available forces in this small area,” the institute said in its latest assessment.
Russia’s Defense Ministry said Russian soldiers and Moscow-backed separatists had also claimed control of Metolkine, just southeast of Sievierodonetsk. The Russian state news agency Tass said that many Ukrainian fighters had surrendered there, although it was not possible to verify the assertions independently.
As the Sievierodonetsk area fell deeper into peril on Sunday, intensifying Russian attacks across much of Ukraine — including renewed shelling near Kharkiv in the north, strikes at Mykolaiv in the south and destruction of infrastructure in the country’s east and central regions — made clear that the war could still flare up far beyond Donbas, where Russia has refocused its military efforts after failing to capture Kyiv in the spring.
Ukraine faces an increasingly grim and bloody fight in its east, where Russia is using long-range artillery to bombard cities and military positions. Ukrainian officials have complained that advanced weaponry from their allies is arriving too slowly to overturn Russia’s firepower advantages and that as many as 200 Ukrainian soldiers are being killed daily.
The slow-moving conflict in Donbas is sapping the morale of both sides, the British defense intelligence agency said on Sunday.
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“Ukrainian forces have likely suffered desertions in recent weeks,” the agency said in its latest public assessment. But it described the problems in Russian ranks as more systemic and severe, including “cases of whole Russian units refusing orders and armed standoffs between officers and their troops.”
“Morale problems in the Russian force are likely so significant that they are limiting Russia’s ability to achieve operational objectives,” it said.
In the northeastern city of Kharkiv, Ukraine’s second-largest, where Russian forces were pushed out several weeks ago, Russian missiles hit a tank repair plant, a Russian military spokesman said on Sunday, destroying what the Russian Defense Ministry said were two Urugan rocket launchers.
Vadym Denysenko, an adviser to Ukraine’s interior minister, said on Sunday that “Russia is trying to make Kharkiv a frontline city,” describing the situation there as difficult.
Russian strikes also targeted a gas processing plant near the eastern city of Izium, causing a large fire, and destroyed an oil depot in the central Dnipropetrovsk region, killing one person and wounding 11 others, Ukrainian officials said.
In the port city of Mykolaiv, which remains in Ukrainian hands, Ukrainian officials said Russian missiles over the weekend had also destroyed businesses and infrastructure. Two people were killed in the village of Halystsynove in the same region in a separate strike on Saturday, an attack that caused a fire, regional officials said.
But it is in the Donbas, an area of rolling plains, farm fields and coal mining towns not far from the Russian border, that Moscow has committed the bulk of its military power in recent weeks. The Sievierodonetsk pocket, as military officials call the area around Sievierodonetsk and Lysychansk, is about three-quarters encircled by Russian forces, leaving only a slender gap to the west where Ukrainian troops come and go by village roads that are often shelled by Russia.
Russian troops have been creeping forward to close the gap. On Sunday in Toshkivka, which has served as an important part of Ukraine’s defensive wall, they appeared to partly succeed.
Ukrainian artillery soon engaged them in intense fighting, Mr. Haidai said, and the outcome was unclear.
Ukrainian battle tanks and several Grad multiple launch rocket systems were seen heading in the direction of the village on Sunday afternoon, smoke billowing from their chassis and treads churning up backcountry roads.
If Russian forces succeed in breaking through the defensive lines, they will reduce Ukrainian troops’ ability to maneuver in the Sievierodonetsk pocket. The advance would also further allow Russian forces to threaten the few remaining supply routes into Lysychansk and Sievierodonetsk, where about 70,000 civilians remain, many of them too elderly or frail to evacuate.
President Volodymyr Zelensky of Ukraine said this month that the “fate” of much of eastern Ukraine was being decided in the battle for the two cities.
Their strategic importance partly explains why Ukraine has chosen to hold out in street fighting there, a strategy fraught with risks. Fighting at close range in Sievierodonetsk, the Ukrainian thinking goes, denies Russia the chance to bring its vast advantage in artillery weapons to bear.
But as Russian forces have encircled soldiers in Sievierodonetsk and supporting troops in Lysychansk, Russian artillery gun lines have pummeled roads, bridges and Ukrainian troop positions with thousands of shells fired every day, by Ukrainian estimates.
Still, the street fighting in the city and battles in the fields around it have cost the Russians dearly in lives and equipment, more than the Ukrainians have lost, Ukrainian commanders have said. After months of fighting, the Russian Army in Ukraine is exhausted and nearing the limits of its resources, Andriy Zagorodnyuk, a former defense minister of Ukraine, said Sunday in an interview.
The Ukrainian approach, he and other analysts said, is to render each advance as costly as possible.
“Despite the precarious position, Ukraine has chosen to fight for these cities in an effort to exhaust Russian forces,” said Michael Kofman, the director of Russia studies at CNA, a research group in Virginia. He said in a recent analysis that if the Russians broke through in Sievierodonetsk, their manpower struggles could still impair their ability to sustain any advance.
The military governor of Luhansk, Mr. Haidai, said that Russian forces were strengthening their reserves in the surrounding area. As intense shelling pounded the cities on Sunday, Mr. Haidai said that Russian forces had bombarded the Azot chemical plant, one of the Ukrainian fighters’ last strongholds in Sievierodonetsk, where hundreds of civilians were also believed to be sheltering.
Under political pressure to produce a victory, said the former defense minister, Mr. Zagorodnyuk, the Russian military is heedlessly throwing resources at the battle.
“They need to show to their leadership that they have achieved something,” he said of the troops and weaponry being hurled into the fight.