Ukrainian forces dug in while Russia lined up more firepower on Monday and tapped a decorated general as war commander ahead of a potentially decisive showdown in eastern Ukraine that experts said could start within days with a full-scale offensive.

The outcome of that confrontation could determine the course of the war, which has flattened cities, killed untold thousands and isolated Moscow economically and politically. Questions remain about the ability of Russia's depleted and demoralised forces to conquer much ground after their advance on the capital, Kyiv, was repelled by determined Ukrainian defenders.

Britain’s Defence Ministry reported on Sunday that the Russian armed forces were trying to compensate for mounting casualties by recalling veterans discharged in the past decade.

Meanwhile, a senior US official said Russia appointed General Alexander Dvornikov, one of its most seasoned military chiefs, to oversee the invasion that Moscow refers to as a “special military operation". The official was not authorised to be identified and spoke on condition of anonymity.

The new battlefield leadership comes as the Russian military prepares for what is expected to be a large, focused push to expand control in the country's east.

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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 the Russian forces that were 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.

US national security adviser Jake Sullivan, speaking Sunday on CNN's State of the Union played down the significance of the appointment.

“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.

Newly released Maxar Technologies satellite imagery showed a 13-kilometre 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 defences, 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 Defence Ministry spokesman, Major General Igor Konashenkov, said Russia's military used air-launched missiles to hit Ukraine’s S-300 air-defence missile systems in the southern Mykolaiv region and at an air base in Chuhuiv, a city not far from Kharkiv.

Sea-launched Russian cruise missiles 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.

The airport in Dnipro, Ukraine’s fourth-largest city, was also hit by missiles twice on Sunday, according to the regional governor.

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

In a late-night video message, Zelensky argued that Russia’s aggression “was not intended to be limited to Ukraine alone”.

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,” Zelensky said.

”This is, in fact, a strategy of defence for every civilised state.”

The Ukrainian leader also thanked British Prime Minister Boris Johnson, who made a surprise visit to Kyiv on Sunday.

Zelensky said they discussed “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 civilians, including airstrikes on hospitals, a missile attack that killed 52 people at a train station and other violence that came to light as Russian soldiers withdrew from the outskirts of Kyiv.

Ukraine has blamed Russia for killing 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 Russian troops retreated. Russia has denied the allegations and falsely claimed that the scenes in Bucha were staged.

Elsewhere, the International Atomic Energy Agency said Ukraine was able to rotate staff at the decommissioned Chernobyl nuclear plant for only the second time since Russian forces seized the facility early in the war.

The nuclear agency said the situation around Chernobyl, site of a 1986 nuclear disaster, “remained far from normal” after Russians departed at the end of March.

Ukrainian officials told the agency Sunday that laboratories for radiation monitoring at the site were destroyed and instruments damaged or stolen.