KYIV, Ukraine — Ukraine’s electricity grid operator warned of hours-long power outages Friday as Russia zeroed in on Ukraine’s energy infrastructure with renewed artillery and missile attacks that have interrupted supply to as much as 40% of the population at the onset of winter.

Grid operator Ukrenergo said outages could last for several hours with colder temperatures putting additional pressure on energy networks.

You always need to prepare for the worst, we understand that the enemy wants to destroy our power system in general, to cause long outages,” Ukrenergo’s chief executive Volodymyr Kudrytskyi told Ukrainian state television Friday. “We need to prepare for possible long outages, but at the moment we are introducing schedules that are planned and will do everything to ensure that the outages are not very long.”

Kudrytskyi added that the power situation at critical facilities such as hospitals and schools has been stabilized.

In the northeastern Kharkiv region, overnight shelling and missile strikes targeted “critical infrastructure” and damaged energy equipment, according to regional governor Oleh Syniehubov. Eight people including energy company crews and police officers were injured trying to clear up the debris, he said.

Moscow’s attacks on Ukraine’s energy and power facilities in the past weeks have left millions without heating and electricity, fueling fears of what the dead of winter will bring. Energy infrastructure had again been targeted Thursday after Russia two days earlier unleashed a nationwide barrage of more than 100 missiles and drones that knocked out power to 10 million people.

Those attacks have also had a knock-on effect on neighboring countries like Moldova where a half-dozen cities across that country experienced temporary blackouts.

Russian forces unleashed the breadth of their arsenal to attack Ukraine’s southeast employing drones, rockets, heavy artillery and warplanes resulting in the death of at least six civilians and the wounding of an equal number in the past 24 hours, the office of the president reported.

In the Zaporizhzhia region, part of which remains under Russian control, artillery pounded ten towns and villages. The death toll from a rocket attack on a residential building in the city of Vilniansk Thursday climbed to nine people, the deputy head of Ukraine’s presidential office Kyrylo Tymoshenko posted on Telegram.

In Nikopol, located across the Dnieper River from the Zaporizhzhia Nuclear Power Plant, 40 Russian missiles damaged several high-rise buildings, private houses, outbuildings and a power line.

In the wake of its humiliating retreat from the southern city of Kherson, Moscow intensified its assault on the eastern Donetsk region where Russia’s Defense Ministry said Friday its forces took control of the village of Opytne and repelled a Ukrainian counteroffensive to reclaim the settlements of Solodke, Volodymyrivka and Pavlivka.

The city of Bakhmut, a key target of Moscow’s attempt to seize the whole of Donetsk and score a demonstrable victory after a string of battlefield setbacks, remains the scene of heavy fighting, said regional governor Pavlo Kyrylenko.

The Russian Defense Ministry also said that Ukrainian troops were pushed back from Yahidne in Ukraine’s eastern Kharkiv province, and Kuzemivka in the neighboring Luhansk province. Donetsk and Luhansk were among the four Ukrainian provinces illegally annexed by Moscow in September, together with Kherson and Zaporizhzhia.

At the same time, Moscow is fortifying its defenses in the southern region to thwart further Ukrainian advances. Russian troops have built new trench systems near the border of Crimea, as well as near the Siversky-Donets River between Donetsk and Luhansk Oblasts, according to a British Ministry of Defense report.

Meanwhile, Ukrainian and international investigators continue their work on uncovering suspected war crimes committed by Russian forces during the Karkhiv region’s near seven month-old occupation. Ukraine’s National Police said in a statement Friday that its officers had initiated over 3,000 criminal proceedings over what it said were “violations of the laws of customs of war” by Russian troops.

A lighting Ukrainian counteroffensive in September reclaimed the Kharkiv region and pushed Russian forces back toward the Donbas, the country’s eastern industrial heartland, and regain strategically located cities including Izium and Kupiansk.

Reports of torture and other atrocities committed by Russian troops have also emerged from the Kherson region where Ukrainian officials said they have opened more than 430 war crimes cases and are investigating four alleged torture sites.

Alesha Babenko, from the village of Kyselivka said he arrested by the Russians in September and locked in a basement. The 27-year-old said he was regularly beaten by Russian soldiers while bound, blindfolded and threatened with electric shocks.

“I thought I was going to die,” he told The Associated Press.

Kherson residents continued to line up for food from a charity with many saying they had nothing to eat and are making do without heating or electricity. One man said “all the fridges have defrosted, we have nothing to eat.”

Despite the hardship, a small sign of a return to normality was news that the first train from the capital Kyiv to Kherson would be departing Friday night. Ukraine’s state rail network Ukrzaliznytsia said around 200 passengers will travel on the train – the first in nine months.

Dubbed the “Train to Victory”, the train’s carriages were painted in eclectic designs by Ukrainian artists and the tickets were sold as part of a “Tickets to Victory” charity project.

In Vienna, the International Atomic Energy Agency’s board of governors approved a resolution calling, among other things, for Russia to withdraw from the Zaporizhzhia plant, Ukraine’s and Europe’s biggest.

British ambassador Corinne Kitsell tweeted that 24 countries voted for and two against the resolution, which was led by Canada and Finland, on Thursday evening. Russian ambassador Mikhail Ulyanov identified the two countries that voted against as Russia and China, and said seven states abstained.