Russian forces are now in control of around 70% of Sievierodonetsk, a strategically important city in Ukraine’s east, Luhansk regional governor Serhiy Gaidai said .

“Some Ukrainian troops have retreated to more advantageous, pre-prepared positions,” Gaidai said on the Telegram messaging app

Russian troops have now taken control of most of Sievierodonetsk, one of two provinces in the Donbas, regional Governor Serhiy Gaidai on Tuesday.

Nearly all critical infrastructure in Sievierodonetsk had been destroyed and 60% of residential property damaged beyond repair, he added. Russian shelling had made it impossible to deliver aid or evacuate people.

A Russian victory in Sievierodonetsk and its twin city of Lysychansk across the Siverskyi Donets river would bring full control of Luhansk, one of two eastern provinces Moscow claims on behalf of separatists.

A pro-Moscow separatist leader said Russian proxies had advanced slower than expected to “maintain the city’s infrastructure” and exercise caution around its chemical factories.

“We can say already that a third of Sievierodonetsk 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.

Gaidai warned Sievierodonetsk residents not to leave bomb shelters due to what he said was a Russian air strike on a nitric acid tank.

The Luhansk People’s Republic’s police force said Ukraine’s forces had damaged it. Ukraine and Russian-backed separatists traded accusations over a similar incident in April.

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.

Up to 12,000 civilians remain caught in crossfire, without sufficient access to water, food, medicine or electricity, Egeland 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,” he said.