Russian forces tried to storm Sievierodonetsk but were unsuccessful and retreated, Zelenskiy's office said. The city, on the Siverskiy Donets River, has been the main Russian target in recent days as Moscow tries to encircle Ukrainian forces and fully capture Luhansk and Donetsk provinces.
With Moscow having captured Mariupol last week after a three-month siege but losing territory elsewhere, the war in Ukraine is entering what some Western military analysts describe as a new phase: a major Russian push to capture the eastern region known as the Donbas, before Moscow is expected to shift to defending and imposing its rule over territory it controls.
In Kyiv, a court handed down a life prison sentence on a Russian soldier who had pleaded guilty to killing a Ukrainian civilian at the first war crimes trial of the conflict. The Kremlin complained that it was not permitted to defend its citizen.
In Mariupol, where hundreds of Ukrainian fighters finally laid down their arms last week after a nearly three-month siege, Russian mine clearing teams were combing through the ruins of the giant Azovstal steel plant.
A huge armoured bulldozer painted with a white letter “Z”, symbol of Russia's assault, pushed debris to the side as a small group of soldiers picked their way through the wreckage with metal detectors.
“The task is huge. The enemy planted their own landmines, we had also planted anti-personnel mines while blocking the enemy. So we've got some two weeks of work ahead of us,” said a Russian soldier, going by the nomme de guerre Babai. He said his crew had destroyed more than 100 explosives over two days so far.
Ukraine has been trying to secure a prisoner swap for fighters who surrendered at the steel plant last week. The leader of pro-Russian separatists in control of the area said they would be tried by a tribunal, but a Russian deputy foreign minister was quoted as saying Moscow could discuss a swap.
An aide to Mariupol's Ukrainian mayor, operating outside the city, said remaining residents were now in danger of disease as sewers overflow amid the ruins. Ukraine believes tens of thousands of people died in the siege of the city of more than 400,000 people.
'WIPING OFF THE FACE OF THE EARTH'
Russia has focused its “special military operation” on the east since its troops were driven out of the area around the capital Kyiv and the north at the end of March.
Since last month, Moscow has said its main effort is capturing all of Donetsk and Luhansk provinces, together known as the Donbas, which Russia claims on behalf of separatists.
Despite pouring its forces into those areas and launching massive artillery bombardments, it has made only small territorial gains, meanwhile continuing to lose territory in a Ukrainian counter-attack further north around Kharkiv.
But the full capture of Mariupol last week gives Russia its biggest victory for months. Its forces now control a largely unbroken swathe of the east and south, freeing up more troops to join the main Donbas fight.
In recent days it has launched a series of assaults to capture Sievierodonetsk, the easternmost part of a Ukrainian-held pocket of the Donbas and one of the last parts of Luhansk province still outside Russia's grip.
Luhansk governor Serhiy Gaidai said Russia was using scorched earth tactics and “wiping Sievierodonetsk from the face of the earth”. Moscow was trying to advance from three directions, to overrun Sievierodonetsk, cut off a highway south of it and cross the river further west.
Some Western military experts say Russia may soon be running out of combat power to wage offensive operations and could have to shift in coming weeks to defending territory. The arrival of more Western weapons would strengthen Kyiv for a future counterattack.
“As their eastern offensive loses momentum, the Russians will inevitably have to transition to a defensive strategy in Ukraine. And in doing so, the Russian Army will confront a new range of difficult challenges ahead,” tweeted Mick Ryan, a retired Australian major general and defence analyst. “The Ukrainian Army will be able to decide where and when it engages the Russians.”
Britain's ministry of defence said Moscow had probably endured losses in three months in Ukraine on par with its losses over nine years in Afghanistan in the 1980s.
“A combination of poor low-level tactics, limited air cover, a lack of flexibility, and a command approach which is prepared to reinforce failure and repeat mistakes has led to this high casualty rate, which continues to rise in the Donbas offensive,” it said.