Smoke and fire erupt from a missile explosion on Ukraine's largest dam, the DniproHES, in Zaporizhzhia on Friday. Reuters

Russia fired almost 90 missiles and more than 60 Iranian-designed kamikaze drones, damaging dozens of energy facilities including power stations, in what Ukrainian officials said was an attempt to cripple the country's electricity and heating network.

Russia said the wave of strikes was retaliation for a series of Ukrainian attacks on its border regions in recent weeks.

At least five people were killed and more than 20 injured, according to Ukraine's interior ministry and local officials.

"We are in a state of war," Kremlin spokesman Dmitry Peskov said in an interview to a pro-Kremlin newspaper, published on Friday.

The admission marks an escalation in official language used to describe the conflict, which the Kremlin initially referred to as a "special military operation." "Yes, it started as a special military operation, but as soon as this bunch was formed there, when the collective West became a participant on Ukraine's side, for us it already became a war," Peskov said.

"De jure (legally) it is a special military operation. But de facto it has turned into a war," he added.

Meanwhile, Ukraine said Russia was preparing 100,000 troops who could be used for a possible new offensive push in Ukraine this summer or to replenish depleted units.

Lieutenant-General Oleksandr Pavliuk made his comments on Ukrainian television after recent advances by Russian troops in eastern Ukraine, more than two years after Moscow's full-scale invasion.

"It will not necessarily be an offensive, perhaps they will replenish their units that lost combat capability but there is a possibility that at the beginning of the summer they may have certain forces to conduct offensive operations," Pavliuk said.

Calling the strikes "retaliation" for recent Ukrainian attacks, Moscow's defence ministry said on Friday it had targeted Ukraine's "energy sites, military-industrial complex, railway hubs and arsenals." "All of the objectives of the massive strike were achieved," it said.

"This morning's attack on Ukraine's energy system was the largest on record," Ukraine's state-run power grid, Ukrenergo, said.

"Dozens of power system facilities have been damaged," including thermal and hydroelectric power plants, as well as major and regional power lines and blackouts had hit seven regions, it added.

Ukrainian President Volodymyr Zelensky appealed once again on Friday for more Western arms, blasting political "indecision" that he said was costing Ukrainian lives.

Ukraine has struggled with shortages of both air defences to protect its skies and ammunition on the ground, as a vital $60 billion military aid package is currently held up in the US Congress.

"Russian missiles do not have delays, as do aid packages to our country. The 'Shahed' (drones) have no indecision, like some politicians. It is important to understand the cost of delays and postponed decisions," he said.

"We need air defence to protect people, infrastructure, homes and dams. Our partners know exactly what is needed. They can definitely support us... Life must be protected from these non-humans from Moscow." Ukraine's air force said it shot down 37 of the 88 missiles fired overnight and 55 of 63 drones.

The strikes left around 700,000 consumers without electricity in the northeastern Kharkiv region, governor Oleg Synegubov said.

"The goal is not just to damage, but to try again, like last year, to cause a large-scale failure of the country's energy system," said energy minister German Galushchenko.

Ukraine’s Prime Minister Denys Shmygal said on Friday: "The situation in the energy sector is under control." Emergency electricity was being supplied from Romania, Slovakia and Poland, Galushchenko said.

The strikes severed one of two power lines supplying the Zaporizhzhia nuclear power station, Europe's largest nuclear power plant in southeast Ukraine that was seized by Russian troops at the start of the war.

The plant has suffered multiple blackouts since the beginning of the war, falling back on emergency diesel generators and safety systems.

"In case of their failure, a threat of a nuclear and radiation accident will emerge," said Energoatom.

Agencies