Russian forces have cut off the last routes for evacuating citizens from the eastern Ukrainian city of Sievierodonetsk, a Ukrainian official has said.
The last bridge to the city was destroyed, trapping any remaining civilians and making it impossible to deliver humanitarian supplies, regional governor Sergei Haidai said.
He said about 70 per cent of the city was under Russian control.
Eduard Basurin, an official of the Russia-backed separatists in Donetsk, said Sievierodonetsk had been blocked off and Ukrainian fighters had no choice but to surrender or die.
Mr Haidai dismissed that as "a lie", saying: "There is no threat of our troops being encircled in the Luhansk region."
"It's hard to deliver weapons or reserves. Difficult, but not impossible," he told Radio Free Europe/Radio Liberty's Ukrainian service.
Ukraine has issued increasingly urgent calls for more Western heavy weapons to help defend Sievierodonetsk, which Kyiv says could hold the key to the battle for the eastern Donbas region and the course of the war, now in its fourth month.
10,000 civilians trapped in Sievierodonetsk
Mr Haidai said fierce street fighting continued on Monday in Sievierodonetsk, one of two large cities in the Donbas region still to be fully captured by Russian troops.
Ukrainian troops were fighting the enemy "block by block, street by street, house by house with a varying degree of success," he said.
More than 10,000 people remain in the city.
Mr Haidai said shelling killed three civilians in the city on Sunday, including a six-year-old boy.
'One of the most brutal battles'
Ukrainian President Volodymyr Zelenskyy warned the battle over the Donbas "will surely go down in military history as one of the most brutal battles in and for Europe".
He vowed Ukraine would liberate all occupied territory, including Crimea, which Russia annexed in 2014.
"The price of this battle for us is very high," he said.
"It's just terrible.
"We draw the attention of our partners daily to the fact that only a sufficient number of modern artillery for Ukraine will ensure our advantage."
The total frontline in the country, he said, was now 2,500 kilometres long.
Earlier, the leader of the Donetsk separatist region asked for additional forces from Moscow to help counter renewed Ukrainian attacks.
Denis Pushilin said there had been increased fighting and shelling in the region and added that "all necessary forces" would be needed to "counter the enemy".
Kremlin spokesman Dmitry Peskov was quoted by Russia's RIA state news agency as saying: "In general, the protection of the republics is the main goal of the special military operation."
RIA reported earlier that several civilians, including a child, died in Ukrainian shelling on Monday.
Reuters was not able to independently verify Russia's claims about increased shelling in the Donetsk region and there was no immediate reaction from Kyiv to the developments.
Russia accused of using cluster bombs against civilians
Amnesty International accused Russia of killing and wounding hundreds of civilians through the indiscriminate use of banned cluster munitions in the city of Kharkiv.
Ukraine's second-largest city has been subject to intensive shelling since Russia began attacking Ukraine.
"The repeated use of widely banned cluster munitions is shocking, and a further indication of utter disregard for civilian lives," said Donatella Rovera, Amnesty International's senior crisis response adviser.
"The Russian forces responsible for these horrific attacks must be held accountable for their actions, and victims and their families must receive full reparations."
The Amnesty report cited doctors in Kharkiv hospitals who showed researchers distinctive fragments they had removed from patients' bodies, as well as survivors and witnesses of the attacks.
Russia says it has destroyed rocket systems
The Russian military said it had destroyed "a large number of weapons and military equipment" sent to Ukraine by the US and Europe.
Defence Ministry spokesman Igor Konashenkov said "high-precision air-launched missiles" hit the supplies near the Udachna railway station in the Donetsk region.
Mr Konashenkov also said "a temporary deployment point for foreign mercenaries" and an anti-aircraft radar station were destroyed in the neighbouring Luhansk region.
Two batteries of multiple-launch rocket systems were destroyed in the Luhansk and Kharkiv regions, Mr Konashenkov said.
There was no immediate confirmation of the Russian claim from Ukraine.
ABC/wires
