KRAMATORSK, Ukraine — It all starts with a whistle, said Vladislav Goncharenko, a Ukrainian army sergeant, describing the relentless Russian shelling.

“You lie in a trench,’’ he said, waiting in an ambulance packed with other wounded soldiers. “There are very loud explosions. You want to get deeper into the ground. And you have shrapnel whistling above you, like flies.”

Soldiers, he said, “just want it to stop.”

Although much of the world’s focus in the war has been on Russia’s disorganized and flawed campaign, Ukraine too is struggling. Ukraine’s army has suffered heavy losses, shown signs of disarray and, step by step, fallen back from long-held positions in Donbas, the eastern region that is now the war’s epicenter.

The momentum that Ukraine generated after pushing Russian forces back from Kyiv, the capital, and Kharkiv, the second-largest city, has given way in the east to weeks of give-and-take over villages, heavy shelling — and a stream of Ukrainian dead and wounded from the battlefields.

Ukraine’s troops now face a Russian force that has shifted strategy from the hasty, reckless advances of the early weeks of the war to a creeping, grinding march enabled by massive artillery bombardments.

On Wednesday, Russian forces advanced in street fighting in the ruins of the city of Sievierodonetsk, a key target of their offensive. A local official said Wednesday that Russian forces controlled about 70% of the city, where only about 12,000 residents remain out of a prewar population of 100,000 after weeks of intense shelling.

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Ukrainian soldiers there are at risk of being surrounded. With bridges over the Seversky Donets River destroyed or under fire, resupply has become tenuous.

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Ukrainian officials have been candid about the army’s travails while arguing more rapid deliveries of Western weaponry will resolve them. Every day in the current heavy fighting, President Volodymyr Zelenskyy said in an interview with Newsmax this week, 60 to 100 Ukrainian soldiers are killed and another about 500 soldiers are wounded in combat.

In his nightly address, Zelenskyy acknowledged the battle for control of the Donbas region was “very difficult” but emphasized that his troops were having success in the south, near Kherson and around Zaporizhzhia, and around Kharkiv in the northeast.

“The front-line situation must be assessed comprehensively,” he said. “Not by one area, where there is the most tough situation and which attracts the most attention, but by the whole front line.”

To fill gaps in the front line, Ukraine has resorted to deploying minimally trained volunteers of the Territorial Defense Force, which mobilized quickly as the war started. Hints of morale lapses have surfaced. One unit recorded a video protesting dire conditions. In interviews, soldiers said their artillery guns sometimes go quiet for lack of ammunition.

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“Those people who said that the war would end very soon, that we have already won, that we will celebrate in April, said a dangerous thing,” Ukraine’s national security adviser, Oleksiy Danilov, told Ukrainian media this week.

In the messy seesaw fighting on the east’s rolling plains, Ukrainian forces are buoyed by the promise of Western weapons arriving soon.

On Tuesday, President Joe Biden announced plans to give Ukraine multiple rocket launch systems, a powerful, long-range artillery weapon. U.S. and Ukrainian officials have said the systems are not intended to strike targets within Russia.

On Wednesday, Chancellor Olaf Scholz of Germany promised to send a sophisticated air defense system and a tracking radar capable of locating Russian artillery. Scholz had faced criticism from Ukraine and some German lawmakers that he had not done enough to support Ukraine’s military. He did not announce a timeline for the new shipments.

With the arrival of new weapon systems weeks away at best, it’s unclear if they will land in Ukraine in time to repel the Russians’ slow advance. Last week, Ukraine was forced from positions it had defended through eight years of war with Russian-backed separatists near the town of Svitlodarsk.

Throughout the war, the state of the Ukrainian military has been difficult to assess from publicly available sources. As the war began, the Ukrainian military had about 30,000 troops deployed in the Donbas region, but neither the government nor the military will provide a current figure.

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The Ukrainian government has largely withheld casualty figures and Western governments have not volunteered their own assessments of the army’s difficulties, as they have in describing Russian setbacks. The last Ukrainian casualty update came April 16, when Zelenskyy said fewer than 3,000 soldiers had died, but his comments about casualties last week suggest the figure is far higher now.

Ukraine is also hampered by the deterioration and depletion of its Soviet-legacy artillery, said Mykhailo Zhirokhov, the author of a book on Ukrainian artillery. The worn barrels fire less accurately. Shells are running low. Western replacements are arriving, but slowly.

The morale of volunteer fighters is also proving to be a challenge, at least in some units. Many who signed up to Ukraine’s Territorial Defense Force in the first days of war believed their task would be limited to defending their hometowns. There were teachers, computer programmers, taxi drivers and others, most with no battlefield experience.

Now they find themselves deployed into vicious combat in the east, an indication of Ukraine’s mounting demand for front-line fighters.

In interviews in ambulances as they were evacuated from the front, about a dozen wounded Ukrainian soldiers said artillery was the cause of most casualties. They echoed appeals of Ukrainian officials for the West to transfer more long-range artillery to counter Russian bombardment.

“It’s a weapon that I, as a rifleman, cannot fight,” Goncharenko said of Russian artillery.

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He was wounded in a barrage on the northern rim of the front around Sievierodonetsk that knocked a tree over the trench he was sheltering in. He suffered a concussion that left him dizzy, vomiting and unable to fight.

The Russians mix artillery barrages with probing maneuvers by infantry or armored vehicles, identifying new targets by approaching Ukrainian lines and drawing fire. The maneuver is called “reconnaissance until contact.”

Ukrainians open fire on the probing Russians, causing casualties. “We collect their dead,’’ Goncharenko said.

But then, having ascertained Ukrainian positions, he said, the Russians “pull back and fire artillery.”

Russia has paid heavy costs as well. On Tuesday U.S. officials estimated that the Russian military’s overall fighting strength had been diminished by about 20%. In late March, NATO estimated that 7,000 to 15,000 Russian troops had been killed.

Still, Russia’s artillery has devastated towns and cities ahead of the advance and prompted about 80% of the population of Ukrainian-controlled areas in the Donbas to flee. Russian soldiers wind up taking ruins.

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“The only way they will occupy Donbas is reducing it to rubble,” said Maria Zolkina, a political analyst. “If they capture Donbas, it will be without cities” or people.

Some military analysts see no clear end for now, and Antony Blinken, the U.S. secretary of state, on Wednesday predicted “many months of conflict” ahead. Russia is unlikely to soon capture the claimed borders of two separatist states whose independence it recognized in February. And Ukraine seems far from ready for a counterattack to turn the tide.

“This is a war where territory is going to change hands, there’s no logical stopping point in the conflict and there’s no stalemate,” said Michael Kofman, the director of Russian studies at CNA, a research institute in Arlington, Virginia, said. “This is going to be a longer war.”