KYIV, Ukraine — Ukrainian forces dug in while Russia lined up more firepower and tapped a general as war commander ahead of an expected showdown in eastern Ukraine that could become decisive in a conflict that has flattened cities, killed untold thousands and isolated Moscow economically and politically.

Experts say a full-scale offensive in the east could start within days, though questions remained about the ability of Russia’s depleted and demoralized forces to conquer much ground after Ukraine’s determined defenders repelled their push to capture the capital, Kyiv.

Britain’s Defense Ministry reported Sunday that Russia’s armed forces were trying to compensate for mounting casualties by recalling personnel discharged in the past decade. The bulk of Ukraine’s military forces are in the east; estimates vary, but they are believed to number in the tens of thousands.

Meanwhile, a senior U.S. official said Russia has appointed Gen. Alexander Dvornikov, one of its most seasoned military chiefs, to command the armed invasion of Ukraine that Moscow refers to as a “special military operation.” The official was not authorized to be identified and spoke on condition of anonymity.

The new battlefield leadership follows Russia’s failed push to capture Ukraine’s capital, Kyiv, decision to prepare for what is expected to be a large, focused push to expand control in the country’s east. Russia-backed separatists have fought Ukrainian forces in the eastern Donbas region since 2014 and declared some territory there as independent republics.

Dvornikov, 60, gained prominence as head of Russian forces that Moscow deployed to Syria in 2015 to shore up President Bashar Assad’s regime amid the country’s devastating civil war. Russian authorities do not generally confirm such appointments, and have said nothing about a new role for Dvornikov, who received the Hero of Russia medal, one of the country’s highest awards, from President Vladimir Putin in 2016.

U.S. national security adviser Jake Sullivan, speaking on American broadcaster CNN’s “State of the Union” program Sunday, played down the possible significance of the general’s appointment on the course of the war.

“What we have learned in the first several weeks of this war is that Ukraine will never be subjected to Russia,” Sullivan said. “It doesn’t matter which general President Putin tries to appoint.”

Western military analysts say Russia’s assault was increasingly focusing on a sickle-shaped arc of eastern Ukraine — from Kharkiv, Ukraine’s second-largest city, in the north to Kherson in the south. However, the airport in Dnipro, Ukraine’s fourth-largest city and located farther west, was hit by missile attacks twice on Sunday, according to the regional governor.

Newly released Maxar Technologies satellite imagery collected Friday showed an 8-mile (13-kilometer) convoy of military vehicles headed south to the Donbas, recalling images of a convoy that got stalled on roads to Kyiv for weeks before Russia gave up on trying to take the capital.

On Sunday, Russian forces shelled government-controlled Kharkiv and sent reinforcements toward Izyum to the southeast in a bid to break Ukraine’s defenses, the Ukrainian military command said. The Russians also kept up their siege of Mariupol, a key southern port that has been under attack and surrounded for nearly 1 ½ months.

A Russian Defense Ministry spokesman, Maj. Gen. Igor Konashenkov, said Russia’s military used air-launched missiles to hit Ukraine’s S-300 air defense missile systems in the southern Mykolaiv region and at an air base in Chuhuiv, a city not far from Kharkiv.

Russia’s sea-launched cruise missiles also destroyed the headquarters of a Ukrainian military unit stationed farther west in the Dnipro region, Konashenkov said. Neither the Ukrainian nor the Russian military claims could be independently verified.

Ukrainian President Volodymyr Zelenskyy appealed for stronger military and political support from the West, including NATO members that have funneled weapons and military equipment to Ukraine since Russia invaded but denied some requests for fear of getting drawn into the war.

In a late night video message, Zelenskyy argued that more than Ukraine’s future was at stake: Russia’s aggression “was not intended to be limited to Ukraine alone” and the “entire European project is a target,” he said.

“That is why it is not just the moral duty of all democracies, all the forces of Europe, to support Ukraine’s desire for peace,” Zelenskyy said. ”This is, in fact, a strategy of defense for every civilized state.”

The Ukrainian leader also thanked British Prime Minister Boris Johnson, who went to Kyiv on a Saturday trip that the U.K. government did not announce in advance. Zelenskyy said they had decided “what help the United Kingdom will provide to the post-war reconstruction of Ukraine,” especially to rebuild the Kyiv region.

Ukrainian authorities have accused Russian forces of committing war crimes against thousands of civilians during the invasion. The alleged crimes took place during airstrikes on hospitals, a missile attack that killed 52 people at a train station in eastern Ukraine on Friday and as Russian soldiers withdrew from the outskirts of Kyiv.

Zelenskyy said that when he and German Chancellor Olaf Scholz spoke by phone Sunday, “we emphasized that all perpetrators of war crimes must be identified and punished.”

Ukraine has blamed Russia for alleged atrocities against civilians in Bucha and other towns outside the capital where hundreds of bodies, many with their hands bound and signs of torture, were found after the Russian troops retreated. Russia has denied engaging in war crimes and falsely claimed that the scenes in Bucha were staged.

After the Russian forces pulled out from the north this week to regroup for the push in the east, firefighters combed through the rubble of buildings to search for victims or survivors. Maria Vaselenko, 77, a resident of Borodyanka, said her daughter and son-in-law were killed, leaving her grandchildren orphaned.

“The Russians were shooting. And some people wanted to come and help, but they were shooting them. They were putting explosives under dead people,” Vaselenko said. “That’s why my children have been under the rubble for 36 days. It was not allowed” to remove bodies.

In Mariupol, Russia was deploying Chechen fighters, reputed to be particularly fierce. Capturing the city on the sea of Azov would give Russia a land bridge to the Crimean Peninsula, which Russia seized from Ukriane eight years ago.

Residents have lacked food, water and electricity since Russian forces surrounded the city and frustrated evacuation missions. Ukrainian authorities think an airstrike on a theater where civilians were sheltering killed hundreds, and Zelenskyy has said he expects more evidence of atrocities to be found once Mariupol no longer is blockaded;

The Institute for the Study of War, an American think tank, predicted Russian forces will “renew offensive operations in the coming days” from Izyum, a town southeast of Kharkiv, in the campaign to conquer the Donbas, which comprises Ukraine’s industrial heartland.

But in the view of the think tank’s analysts, “The outcome of forthcoming Russian operations in eastern Ukraine remains very much in question.”

Ukrainian officials have pleaded with Western powers almost daily to send more arms and further punish Moscow with sanctions, including the exclusion of Russian banks from the global financial system and a total EU embargo on Russian gas and oil.

In an interview with The Associated Press inside his heavily guarded presidential office complex on Saturday, Zelenskyy said he was committed to negotiating a diplomatic end to the war even though Russia has “tortured” Ukraine.

He also acknowledged that peace likely will not come quickly. Talks so far have not included Putin or other top Russian officials.

“We have to fight, but fight for life. You can’t fight for dust when there is nothing and no people. That’s why it is important to stop this war,” the president said.