Ukrainian forces are still defending its position in the eastern Ukrainian city of Severodonetsk that is key to Moscow’s efforts to quickly complete the capture of the Donbass region on Tuesday, resisting Russia's all-out assault on a bombed-out wasteland.

Both sides said Russian forces now controlled between a third and half of the city. Russia's separatist proxies acknowledged that capturing it was taking longer than hoped, despite one of the biggest ground assaults of the war.

Western military analysts say Moscow has moved manpower and firepower from across the rest of the front to concentrate on Severodonetsk, hoping a massive offensive on the small industrial city will deliver something Russia can call a victory in one of its stated aims in the east.

"We can say already that a third of Severodonetsk is already under our control," Russia's TASS state news agency quoted Leonid Pasechnik, the leader of the pro-Moscow Luhansk People's Republic, as saying.

Fighting was raging in the city, but Russian forces were not advancing as rapidly as might have been hoped, he said, claiming that pro-Moscow forces wanted to "maintain the city's infrastructure" and were moving slowly because of caution around chemical factories.

Ukraine says Russia has destroyed all of the city's critical infrastructure with unrelenting bombardment, followed by wave after wave of mass ground assault involving huge numbers of casualties.

The Ukrainian head of the city administration, Oleksandr Stryuk, said the Russians now controlled half of the city. "Unfortunately ... the city has been split in half. But at the same time the city still defends itself. It is still Ukrainian," he said, advising those still trapped inside to stay in cellars.

"The city is essentially being destroyed ruthlessly block by block," Stryuk said. He said heavy street fighting continues and artillery bombardments threaten the lives of the estimated 13,000 civilians still sheltering in the ruined city that once was home to more than 100,000.

"Civilians are dying from direct strikes, from fragmentation wounds and under the rubble of destroyed buildings, since most of the inhabitants are hiding in basements and shelters," Stryuk said.

Thousands of residents remain trapped. Russian forces are advancing toward the city center and have not succeeded in encircling the Ukrainian defenders holding out there.

Regional governor Serhiy Gaidai told Ukrainian television there did not appear to be a risk of Ukrainian forces being encircled, though they could ultimately be forced to retreat across the Seversky Donets river to Lysychansk, the twin city on the opposite bank.

Stryuk, head of the city administration, said evacuating civilians was no longer possible. Authorities canceled efforts to evacuate residents after an attack on Monday that killed a French journalist.

2 Russian soldiers convicted

A court in Ukraine has convicted two Russian soldiers of war crimes for the shelling of civilian buildings and sentenced both to 11 1/2 years in prison. Tuesday's verdict concluded the country's second war crimes trial since the Russian invasion started.

Russian soldiers Alexander Bobykin and Alexander Ivanov were charged with violating the laws and customs of war over the shelling of civilian infrastructure in the Kharkiv region on the first day of the Russian attack on Ukraine. They both stood trial in a court in Ukraine's Poltava region and pleaded guilty to the charges.

Earlier this month, a court in Kyiv sentenced a 21-year-old Russian soldier to life in prison for fatally shooting a Ukrainian civilian in the first war crimes trial since Russia invaded.

Jan Egeland, secretary general of the Norwegian Refugee Council aid agency, which had long operated out of Severodonetsk, said he was "horrified" by its destruction.

"We fear that up to 12,000 civilians remain caught in crossfire in the city, without sufficient access to water, food, medicine or electricity. The near-constant bombardment is forcing civilians to seek refuge in bomb shelters and basements, with only few precious opportunities for those trying to escape," he said.

Elsewhere on the battlefield, there were few reports of major action on Tuesday. In the east, Ukraine says Moscow is trying to assault other areas along the main front, including pressing toward the city of Sloviansk. In the south, Ukraine claimed in recent days to have pushed back Russian forces on a bank of the Inhulets River that forms a border of Russian-held Kherson province.

Putin launched his invasion of Ukraine in February claiming Moscow aimed to disarm and "denazify" its neighbour. Ukraine and its Western allies call this a baseless pretext for a war to seize territory.