Ukraine insisted Saturday that its forces were fending off relentless Russian attacks in Bakhmut, even as Western analysts said that Moscow’s forces had captured most of the embattled city’s east and established a new front line cutting through its center.

Gradual Russian advances and a high number of Ukrainian casualties have fueled talk of a retreat from Bakhmut, a city in the Donbas region of eastern Ukraine that has been devastated by months of fighting. But Ukrainian officials say that Russian losses in Bakhmut are worse than their own, and they have signaled that they will pursue a strategy of bleeding the Russian army before a planned Ukrainian counterattack.

Despite the Ukrainian military’s assertion that it was holding on in Bakhmut, it was becoming increasingly clear that its grip on the city was tenuous and Russian forces were making new gains. Although Bakhmut’s strategic value is debatable, Moscow is looking for a victory after a series of setbacks.

Yevgeny Prigozhin, the head of Russia’s Wagner mercenary group, said this past week that his fighters had seized the eastern half of Bakhmut – an assertion that Ukraine’s military rejected at the time, saying that its soldiers were still fighting there.

But Britain’s defense intelligence agency said Saturday that over the past four days, Wagner fighters had “taken control” of most of the city’s east. The Bakhmutka River, which runs north to south through the city’s center, now marks the front line and could stymie further Russian advances west, it added.

Satellite images showed that bridges across the Bakhmutka, which before the war was lined by well-kept vegetation and bustling walking paths, had been destroyed. Ukraine had earlier blown up pontoon crossings to prevent Russian advances over the river – and appeared to now be using it as a new defensive line, the British agency said.

“With Ukrainian units able to fire from fortified buildings to the west, this area has become a killing zone, likely making it highly challenging for Wagner forces attempting to continue their frontal assault westward,” it said, noting that Ukrainian forces were still vulnerable amid continued Russian efforts to encircle them.

That assessment was largely echoed by the Institute for the Study of War, a research group in Washington, which said Friday evening that Russian forces had “made gains” in Bakhmut and were clearing the eastern part of the city.

Ukraine’s military said in a statement Saturday that its troops were “giving a decent rebuff” to Russian forces and still holding on to the city. It said that the commander of Ukraine’s ground forces, Col. Gen. Oleksandr Syrsky, was at “the most important area” of the front line and taking the “necessary measures to keep Bakhmut under Ukrainian control.”

Russian forces fired on Ukrainian positions around Bakhmut 157 times over the previous 24 hours, a spokesperson for Ukraine’s eastern command, Serhiy Cherevaty, said Saturday afternoon on national television. The city itself was attacked 16 times, he added, and there were 23 “combat engagements” within it.

Ukrainian and Russian officials have suggested that the fall of Bakhmut could help pave the way for Moscow’s forces to make a broader push in eastern Ukraine. With Ukraine expected to launch its own offensive in the coming weeks, Syrsky made clear Saturday that defending Bakhmut was key to those efforts.

“It is necessary to gain time to accumulate reserves and start the spring counteroffensive, which is not far off,” he said in the statement.

That campaign will probably focus on the south, according to military analysts and Ukrainian officials, who have suggested that Ukraine may try to approach the Russian-held port of Melitopol and drive a wedge between Moscow’s forces in the Crimean Peninsula and those in eastern Ukraine.

In the meantime, rather than withdraw from Bakhmut as had been rumored, President Volodymyr Zelenskyy has said that Ukraine will send reinforcements. That message was underscored late Friday when Zelenskyy again discussed Bakhmut “and our opportunities to strengthen there” with his military leadership, according to a statement from the presidency.

The battle for Bakhmut has been the longest sustained Russian assault since the invasion last year, with a staggering casualty toll for both sides. In recent weeks, both have tried to justify their losses in a minor city of limited strategic value by presenting them as benefiting their cause. Each makes essentially the same assertion: that the fighting there is worth the cost because it is wearing down the opponent and depleting whatever resources might be available to push forward elsewhere.

Russia has poured enormous resources into the fight, including sending waves of former prison inmates enlisted by Wagner in near-suicide assaults. Western officials estimate that up to 30,000 Russian soldiers have been killed or wounded in and around Bakhmut. Ukrainian casualties are also believed to be high, although Western officials refuse to give estimates. Kyiv’s forces are tearing through ammunition supplies – firing shells and rockets far faster than Western nations can supply them.

Ukraine’s Western allies have been rushing to deliver new weapons, including tanks. On Friday, President Joe Biden and Ursula von der Leyen, the president of the European Commission, met in Washington and in a joint statement reaffirmed their “unwavering support for Ukraine for as long as it takes.”

The government in Kyiv has cited improving its air defenses as a priority, with Moscow launching one of its broadest aerial attacks in weeks Thursday. Norway said Friday that it would provide two more air defense systems known as NASAMs to Ukraine in cooperation with the United States.

As Ukraine has looked to the West, Russia has increasingly turned to its own allies in other parts of the world. U.S. officials have expressed mounting concerns over the possibility that Russia, isolated by Western sanctions, is deepening its military relationship with Iran, pointing to the provision of attack drones and shipments of artillery rounds, allegations that Iran has rejected.

Late Friday, the Iranian state news media confirmed a report by the news organization Semafor that Tehran had finalized a deal to buy fighter jets from Russia, a prospect that Biden administration officials had warned could affect both the Ukraine war and security in the Middle East. Iran’s state-run IRNA news agency reported that the agreement “has nothing to do with the Ukraine war, as there is absolutely no military cooperation between Iran and Russia in Ukraine.”

While the battle for Bakhmut has ground on, Russian attacks elsewhere in Ukraine have continued unabated. In the Kherson region, Ukrainian officials said that Russian shelling Saturday killed three people and wounded two others.

This article originally appeared in The New York Times.