Ukraine is switching to emergency shutdowns to stabilise its power grid after a fresh wave of Russian missile attacks hit the country on Monday.
President Volodymyr Zelensky said many regions were affected, and officials said half of the Kyiv region would go without electricity in the coming days.
Four people were killed in Monday’s attacks on Ukraine’s infrastructure.
And overnight more missiles hit critical facilities near the southern city of Zaporizhzhia, officials said.
In a separate development on Tuesday, the governor of Russia’s Kursk region said a drone attack on an airfield set an oil storage tank alight. Videos showed fierce flames and dense black smoke billowing from the site.
That came after a series of explosions at two military airfields deep inside Russia on Monday, which Moscow blamed on Ukrainian drones.
In Ukraine, the energy minister said he hoped to significantly reduce the power deficit caused by the latest Russian strikes by bringing nuclear power stations back onto the grid.
The country is now seeing snow and sub-zero temperatures in many regions, and millions are without electricity and running water, raising fears people may die of hypothermia.
Marina, a resident of Vyshgorod, in the Kyiv region told the BBC how she gathers with her husband and children in one room to keep warm during scheduled power outages.
“We turn on electric heaters and warm up the room well, and then we are all in this room together, without opening the door. We have enough heat for four hours,” the 39-year-old said.
She described how she tries to do everything she can while the lights are working, as they can go out for long periods.
“I’d rather sit here in the cold than leave,” she told the BBC.
President Zelensky said 70 Russian missiles were fired across the country on Monday but that most of them were shot down.
But Ukraine’s leader said that there were power shutdowns in regions spanning the country.
Electricity supplies were also affected in neighbouring Moldova, he said, adding that Russia’s actions were “a threat not only to Ukraine, but also to our entire region”.
The Russian defence ministry claimed it had carried out massive high-precision strikes on Ukraine’s military command and control systems, and on other targets.
Monday’s strikes were Russia’s eighth massive missile attack in eight weeks.
They came hours after a series of explosions at two military airfields housing Russian strategic bombers deep inside Russia, which Moscow blamed on Ukrainian drones intercepted by Russian air-defences.
Three servicemen were killed and two aircraft were lightly damaged at the airfields in the Ryazan and Saratov region. Ukraine has not commented.
Meanwhile, President Vladimir Putin chaired a meeting of Russia’s security council on Tuesday – a meeting that usually happens on Fridays.
State TV showed a clip from his opening remarks, in which the Kremlin leader said the subject of the meeting would be state security.
Before the latest Russian strikes, officials in Kyiv were talking about moving from highly disruptive emergency blackouts, which often last for many hours, to more manageable scheduled power cuts which offer civilians some badly needed predictability.
Moscow has been battering Ukraine’s power grid since 10 October, following a string of heavy military defeats on the battlefield.
Some Western leaders have called the strategy a war crime, because of the huge amount of damage caused to civilian infrastructure.
Experts have told the BBC that Russia’s tactic of hitting energy infrastructure is most likely designed to demoralise and terrorise the population, rather than gain any concrete military advantage.
Moscow has repeatedly denied the allegations.