An incident at a chemical plant producing nitric acid in the embattled eastern Ukrainian city of Sievierodonetsk is raising alarm, with reports of a large cloud of potentially toxic smoke.
Ukrainian authorities, who remain in control of parts of the city, spoke of a Russian airstrike on the plant.
Pro-Russian separatists said there had been an explosion.
In photos published by the governor of the Luhansk region, Serhiy Gaidai, on his Telegram news channel, a large cloud of smoke could be seen.
Separatist representative Rodion Miroshnik said that chemicals had been blown up at the Azot plant.
It appeared to be nitric acid.
The plant is one of the largest chemical companies in Ukraine.
At one time, 7000 people were employed there.
An injured woman seen inside a centre for distribution of humanitarian aid in Sievierodonetsk. Source: Getty / SOPA Images/SOPA Images/LightRocket via Gett
Mr Gaidai said most of Sievierodonetsk was now under Russian control.
Nevertheless, the Ukrainian defenders were not giving up, the governor said.
A total of 90 per cent of the buildings in the city were damaged and 60 per cent were not worth rebuilding, he said.
Mr Gaidai said Russian shelling had made it impossible to deliver humanitarian supplies or allow people to leave.
Russia, responding to sanctions after its 24 February invasion of Ukraine, has widened its gas cuts to Europe on Tuesday with Gazprom saying it would cut supplies to several "unfriendly" countries which have refused to accept Russia's roubles-for-gas payment scheme.
European Union leaders agreed to cut imports of Russian oil by 90 per cent by year-end, the bloc's toughest yet response.
Ukrainian President Volodymyr Zelenskyy welcomed the move but criticised what he called an "unacceptable" delay.
"When over 50 days have passed between the fifth and sixth sanction packages, the situation is not acceptable for us," Mr Zelenskyy said, speaking alongside Slovakian President Zuzana Caputova in Kyiv.
Foreign military analysts say Russia has drained manpower and firepower from across other parts of the front to concentrate on Sievierodonetsk, hoping a massive offensive will achieve one of its stated aims - to secure surrounding Luhansk province for separatist proxies.
An injured woman receives first aid at a centre for distribution of humanitarian aid in Sievierodonetsk after the city was hit by Russian shelling. Source: AAP / SOPA Images/Sipa USA
In its evening briefing note on Facebook, Ukraine's military command said that Russian forces were "attempting to take full control of Sievierodonetsk" and surround Ukrainian units fighting there.
"Unfortunately... the city has been split in half. But at the same time the city still defends itself. It is still Ukrainian," the head of the city administration, Oleksandr Stryuk, said, advising those still trapped inside to stay in cellars.
Ukraine says Russia has destroyed all of the city's critical infrastructure with unrelenting bombardment, followed by wave after wave of mass ground assaults involving huge numbers of casualties.
Jan Egeland, secretary general of the Norwegian Refugee Council aid agency which had long operated out of Sievierodonetsk, 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," he said.
"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."
Elsewhere on the battlefield, there were few reports of major shifts.
In the east, Ukraine says Russia is trying to assault other areas along the main front, regrouping to press towards the city of Sloviansk.
In the south, Ukraine claimed in recent days to have pushed back Russian forces to the border of Russian-held Kherson province.
The EU said it will ban imports of Russian oil by sea.
Officials said that would halt two-thirds of Russia's oil exports to Europe at first, and 90 per cent by the end of this year as Germany and Poland also phase out imports by pipeline.
But Hungary, which relies on Russian oil through a huge Soviet-era pipeline, secured an exemption.
Mr Putin launched his "special operation" in February to disarm and "denazify" Ukraine.
Ukraine and its allies call this a baseless pretext for a war to seize territory.
Ukraine accuses Russia of war crimes on a huge scale, flattening cities and killing and raping civilians.
Russia denies the accusations.
In the second war crimes trial to be held in Ukraine, two Russian soldiers were jailed on Tuesday for 11 and a half years after pleading guilty to shelling civilian targets.
The country's top prosecutor said Ukraine has identified more than 600 Russian war crime suspects and started prosecuting about 80.
