DONETSK REGION, Ukraine — Ukrainian and Russian forces traded fresh blows on Sunday near Sievierodonetsk, military authorities and analysts said, as Moscow renewed its push toward the city, one of the last major Ukrainian strongholds in a key part of the east.
The battle for Sievierodonetsk has emerged as another crucial point in the war, as Russia struggles to notch victories. Following its failed assaults on the capital, Kyiv, and the country’s second largest city, Kharkiv, Russia’s military has regrouped and now appears concentrated on capturing the Donbas region in Ukraine’s east. A victory in Sievierodonetsk would give Russian forces control of Luhansk, one of two provinces that make up the region.
Moscow has already sustained heavy losses in its push toward the city, but capturing it could allow its forces to mount an assault on Kramatorsk, Ukraine’s regional military command to the west. At the same time, Ukraine’s Western allies are racing Howitzers and other long-range weapons to the front line to bolster the resistance.
The ongoing fight is also a sign of Moscow’s narrowing military objectives as the war reaches the three-month mark. Capturing all of Ukraine at once has proved out of reach for Moscow, but Russian forces have found success in slowly chipping away at the country, working their way east to west.
In Sievierodonetsk on Sunday, Russian forces attempted to breach the city’s defenses from four directions. But neither side has been able to move the front line substantially in its favor throughout the chaotic battlefield landscape, which is dominated by farmland and small mining towns and villages that are mostly deserted.
Serhiy Haidai, the head of the Ukrainian military administration in Luhansk, said Russian forces retreated to their previous positions, repelled by Ukrainian forces. The Russian military continued to fire mortar shells at residential areas of Sievierodonetsk, damaging at least seven houses.
Earlier on Sunday, Mr. Haidai said that Ukraine’s National Guard forces had destroyed a piece of heavy artillery, a Pion, that Russian troops had used to shell Sievierodonetsk and to destroy a bridge connecting it to the city of Lysychansk on the other side of the Seversky Donets river.
Mr. Haidai said that Russian propagandists had bragged about the weapon’s whereabouts, enabling the city’s defenders to target it more precisely. “The punishment was quick to come,” he wrote on Telegram, a messaging app.
Ukraine’s military said it had also destroyed Russian vehicles and a pontoon bridge over the Seversky Donets near the town of Serebrianka, which lies about 20 miles west of Sievierodonetsk. A Ukrainian military statement described the Russian plan to cross the river as “mission impossible.”
The 650-mile-long river, which originates in Russia and meanders southeast through the Donbas region, has presented a significant natural obstacle to Russia’s offensive. Some of the invasion force’s biggest losses of the war so far have come during an attempt to cross the river this month.
In a sign of the offensive’s importance to Moscow’s strategic planners, Russia has deployed a company of Terminator armored vehicles to the fighting that were part of the failed offensive against Kyiv, according to a British military intelligence report released on Sunday.
“However, with a maximum of 10 Terminators deployed, they are unlikely to have a significant impact on the campaign,” the report said.
The Institute for the Study of War, a Washington research body that tracks the conflict, said on Saturday that Russian forces had “intensified efforts to encircle and capture Sievierodonetsk,” an effort that was likely to continue because their advances remained largely stalled elsewhere in the Donbas region.
Russian forces in the city of Izium, farther west, have been trying to push south into the Donbas region for weeks, but their offensive has been stalled by stiff Ukrainian resistance.
For weeks, Ukrainian and Russian troops have been engaged in a grueling war of attrition, often fighting fiercely over small areas. One village may fall into Russian hands one day, only to be retaken by the Ukrainians a few days later.
On Sunday, Ukraine’s military said Russian forces had attacked several places along the front line in the east. In an assessment of the war published early Sunday evening, it described intensive artillery fire on mostly deserted towns and villages.
To the north of the city of Sloviansk in Donetsk, which Russian forces have sought to capture, Russian artillery bombarded Ukrainian positions and staged probing assaults that were rebuffed, the Ukrainian military’s report said.
Farther east, Russian forces attacked two frontline villages — Prudnovka and Aleksandrovka — with mortar and artillery fire but also failed to advance, according to the assessment.
On Sunday, Ukrainian troops at positions south of Izium kept watchful eyes on the front line as artillery and mortar rounds pierced the sky.
“They are trying bit by bit, all the time,” said Oleh, 56, commander of a unit of volunteers south of Izium, who asked that their frontline position not be identified precisely, according to military protocol.
“But we are holding,” he added.
Ukrainian troops have held positions south of Izium for two months, he said, adding that he was confident of staving off further attacks as long as Western military assistance kept coming.
“We are ready for anything,” Oleh said, “but we need more heavy weapons, and on this we rely on our allies.”
As the war approaches its fourth month, Western nations have substantially increased their aid to Ukraine, an effort to tip the balance as the conflict grows more protracted and costly.
Russia-Ukraine War: Key Developments
Late last week, the U.S. Senate approved some $40 billion in aid to Ukraine, including lethal assistance, and it was signed by President Biden on Saturday. The package, combined with aid approved in March, amounts to the largest package of foreign assistance passed by Congress in at least two decades.
Russia’s government has warned that Western nations will pay unspecified consequences for aiding Ukraine. On Saturday Moscow’s Defense Ministry claimed that it had struck a military depot west of Kyiv filled with Western military equipment, an assertion that Ukraine’s government did not address.
On Sunday, Jake Sullivan, Mr. Biden’s national security adviser, said he could not confirm whether the weapons depot had been hit because he had not had the chance to consult Ukraine’s government.
“What I can say is that we have what we believe to be a diverse and resilient supply chain for these weapons in Ukraine,” Mr. Sullivan said.
“So even if there is a circumstance where the Russians are able to target and hit some shipping on the ground in Ukraine, that’s not going to fundamentally, from a strategic perspective, disrupt the military assistance we’re providing,” he said.
In a visit to Kyiv on Sunday, Poland’s president, Andrzej Duda, said that Ukraine alone should determine its future and that the international community must demand Russia’s complete withdrawal.
It would be a “huge blow not only for the Ukrainian nation, but also for the entire Western world,” Mr. Duda said, if even a tiny part of Ukraine were sacrificed in a peace deal.
“Worrying voices have appeared, saying that Ukraine should give in to Putin’s demands,” Mr. Duda said of the Russian president in what was the first address by a foreign leader to Ukraine’s Parliament since the war began. “Only Ukraine has the right to decide about its future.”
Mr. Duda’s remarks came as the German, French and Italian governments have suggested a cease-fire, calls that Ukraine has rejected angrily as selfish and poorly timed. Ukrainian officials — backed by some eastern European governments — say that Russia is hardly ready for serious peace talks and must be dealt a decisive blow to end the conflict once and for all. Kyiv asserts that its forces have the momentum in the war, despite considerable losses.
Carlotta Gall reported from the Donetsk region, in Ukraine; Matthew Mpoke Bigg from Krakow, Poland; and Maria Abi-Habib from Mexico City. Andrew E. Kramer contributed reporting from Pokrovsk, Ukraine, and Zolan Kanno-Youngs from Tokyo.