Later in his nightly video address, Zelenskiy said he was speaking to Western leaders, including NATO Secretary General Mark Rutte, British Prime Minister Keir Starmer and German Chancellor Olaf Scholz, to forge a response to "Russia's attempt to make the situation more unbearable and drag out the war".

"Now is the time to strengthen our positions - the position of Ukraine and our partners," he said.

US President Joe Biden condemned the Russian attack as "outrageous," saying in a statement that it serves as a "another reminder of the urgency and importance of supporting the Ukrainian people in their defence against Russian aggression."

More than one million people lost power in the immediate aftermath of the strikes, and millions more had their existing schedule of rolling power cuts intensified.

Ukraine's air force said Russia used 91 missiles and 97 drones in Thursday's attack. It said 12 of those had hit their targets, most of which were energy and fuel facilities.

"The enemy is using a large number of missiles and drones. Their massive use in certain areas often exceeds the number of means of (air defence) cover," the air force said in a statement.

Infrastructure facilities were damaged in nine regions, Ukraine's Interior Ministry said.

The attack reinforced fears of long power cuts during the winter months as temperatures hover around zero.

Officials said it was the 11th major strike on the energy system since March. Russia has knocked out about half Ukraine's available generating capacity during the war, damaged the distribution system and forced authorities to impose long blackouts.

The air force said it had shot down 79 missiles and downed 35 drones, while 62 drones were "lost", meaning it was likely they had been disrupted by electronic warfare.

A source in the energy sector said Ukraine had disconnected all nuclear power units from the grid before the attack to protect them. Ukraine gets more than half of its electricity from nuclear plants.

Ukraine's state grid operator Ukrenergo announced deep power cuts across the country because of damage from the attacks, warning of at least 12 hours without electricity for some consumers.

All missiles or drones aimed at the capital Kyiv were brought down, officials said.

The air force said Russia was using thermal and radar decoys to trick Ukrainian air defences, and putting electronic warfare devices on its missiles.

"All this significantly complicates the operation of Soviet-made anti-aircraft missile systems ... Western systems work much more effectively in such conditions, but Ukraine does not have enough to reliably cover hundreds of critical infrastructure facilities," it said.

It said foggy weather conditions also made it harder for machine gunners to spot drones.

More than 33 months after Russia invaded Ukraine, Russian ground forces are advancing at their fastest pace in two years.

Russia fired a new hypersonic intermediate-range ballistic missile at Ukraine this month after the United States allowed Kyiv to strike Russian territory with advanced Western missiles.

"Putin does not want peace. We must force him into peace through strength," Foreign Minister Andrii Sybiha said, reiterating Kyiv's call for more air defence and long-range capabilities from Western allies.

In western Ukraine, Lviv regional governor Maksym Kozytskyi said Thursday's Russian strikes had cut off electricity to about 523,000 people. Power was also cut to nearly 500,000 people in the Volyn and Rivne regions, their governors said, and disrupted in the Khmelnytskyi and Zhytomyr regions.

State oil and gas firm Naftogaz said its facilities had been attacked in the morning airstrikes.

Officials across Ukraine said they were turning on generators to ensure emergency heat and water supplies to hospitals, schools and other critical facilities during bitter winter weather.

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