KYIV, Ukraine — Ukraine’s president replaced his top general Thursday in a shake-up aimed at reigniting momentum in the deadlocked war with Russia, which is grinding into its third year as the country grapples with shortages of ammunition and personnel and struggles to maintain support from the West.

After days of speculation that change was coming, Ukrainian President Volodymyr Zelenskyy said on social media that he was thankful for the service of the outgoing Gen. Valerii Zaluzhnyi — a military leader popular with troops and the general public. “The time for ... a renewal is now," Zelenskyy said on X.

Zelenskyy appointed the commander of Ukraine’s ground forces, Сol. Gen. Oleksandr Syrskyi, to lead the army, which needs a morale boost at a time when the conflict with Russia has been at a near stalemate for months. Syrskyi, 58, has been involved since 2013 in the Ukrainian army’s effort to adopt NATO standards.

Zaluzhnyi said in a Telegram message that “everyone must change and adapt to new realities” and he agreed there is a “need to change approaches and strategy” in the war.

Neither Zelenskyy nor Zaluzhnyi provided any detail about what the new strategy might entail.

Mykhailo Podolyak, an adviser to Zelenskyy, said Ukraine needed a rethink of its tactics, “which did not fully ensure proper results last year.”

Ukraine needs to “prevent stagnation on the front line, which negatively affects public sentiment, to find new functional and high-tech solutions that will allow (Ukraine) to retain and develop the initiative,” he said on X.

Ukraine’s struggles with ammunition and personnel come on the heels of a failed counteroffensive last summer and as European allies try to bump up their military production. At the same time, a political standoff in the United States is holding up further aid from Ukraine’s main supplier.

Zelenskyy's announcement came as the Kremlin’s forces push harder to take the eastern Ukraine city of Avdiivka, throwing more troops into the four-month battle and bombarding Ukrainian defenses, as they stretch Kyiv’s resources.

Before Thursday's announcement, local media had speculated for days that Zelenskyy would sack Zaluzhnyi. It is the most significant shake-up of military leadership since Russia's full-scale invasion of Ukraine on Feb. 24 2022.

Zaluzhnyi was highly regarded by his troops and by foreign military officials. Some analysts warned that his exit could bring unwelcome disruption, potentially driving a wedge between the Ukrainian army and politicians, and fueling uncertainty among Kyiv’s Western allies.

There has been little change in positions along the 1,500-kilometer (900-mile) front line over the winter, though the Kremlin’s forces have kept up their attacks at certain points. Faced with a shortfall in anticipated supplies of Western weaponry, Ukraine has been digging defenses, while Moscow has put its economy on a war footing to give its military more muscle.

Strains had appeared between Zaluzhnyi and Zelenskyy — arguably the two most prominent figures in Ukraine’s fight — after the much-anticipated counteroffensive failed to meet its goal of penetrating Russia’s deep defenses. Kyiv’s Western allies had poured billions of dollars’ worth of military hardware into Ukraine to help it succeed.

Months later, amid signs of war fatigue in the West, Zaluzhnyi described the conflict as being at a “stalemate,” just when Zelenskyy was arguing in foreign capitals that Ukraine’s new weaponry had been vital.

Zelenskyy said at the end of last year that he had turned down the military’s request to mobilize up to 500,000 people, demanding more details about how it would be paid for.

Born into a family of Soviet servicemen, Zaluzhnyi is credited with modernizing the Ukrainian army along NATO lines. He took charge seven months before Russia’s full-scale invasion.

Widely regarded in the West as an ambitious and astute battlefield commander, he has had a reputation for modesty in Ukraine.

Zaluzhnyi earned broad public support after the successful defense of Kyiv in the early days of the war, followed by a triumphant counteroffensive in the Kharkiv region and the liberation of Kherson. His courage and defiance of Russia’s ambitions were renowned, and he became a symbol of resilience and national unity.

“We are on our land and we will not give it up,” Zaluzhnyi said on the first day of the war.

Despite his popularity, Zaluzhnyi shied from the spotlight, deferring that role to Zelenskyy. He made limited public appearances and rarely gave interviews.

Retired Australian Maj. Gen. Mick Ryan, a fellow of the Center for Strategic and International Studies think tank in Washington, described Zaluzhnyi as “a charismatic and popular military leader” who would be hard to replace.

His replacement will have to build personal relationships with U.S. and NATO military chiefs, while the perception of government instability “is a real danger area for” Zelenskyy, Ryan wrote recently in an article posted online.

Earlier Thursday, Ukrainian forces claimed to have shot down a Russian attack helicopter in eastern Ukraine near the city of Avdiivka, where soldiers are fighting from street to street as Russia's army seeks to surround Kyiv’s defending troops.

Avdiivka has become “a primary focus” of Moscow’s forces, the U.K. Defense Ministry said in an assessment Thursday.

The General Staff of Ukraine's armed forces reported Thursday that its troops had fended off 40 enemy assaults around Avdiivka over the previous 24 hours. That is roughly double the number of daily Russian assaults at other points along the front line.

Ukraine has built multiple defenses in Avdiivka, complete with concrete fortifications and a network of tunnels. Despite massive losses of personnel and equipment, Russian troops have slowly advanced since October.