Russia admitted two years into its invasion of Ukraine on Friday that it was "in a state of war," as it launched a massive wave of missile and drone attacks on its neighbour's territory.

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 supply.

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 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.

Moscow often accuses the West of direct participation in the conflict by supplying Ukraine with weapons.

Calling Friday's strikes "retaliation" for recent Ukrainian attacks, Moscow's defence ministry said 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.

- 'Largest on record' -

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

It said "dozens of power system facilities have been damaged," including thermal and hydroelectric power plants.

Hundreds of thousands were left without electricity in at least three regions.

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."

- 'Under control' -

Ukraine's air force said it shot down 37 of the 88 missiles fired overnight and 55 of 63 drones.

The strikes completely cut electricity and heating supply in the northeastern city of Kharkiv, its mayor said. At least 200,000 in the western Khmelnytsky region and around 260,000 in the southern region of Odesa were also facing power outages.

"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.

Last winter Russia launched a daily barrage of aerial attacks on Ukraine's power grid, plunging millions into darkness and leaving them without heating in sub-zero temperatures for hours.

But the country's energy network has largely held up this year.

Despite the power outages, Prime Minister Denys Shmygal said Friday: "The situation in the energy sector is under control."

Emergency electricity was being supplied from Romania, Slovakia and Poland, Galushchenko said.

  Summer offensive 

 The strikes temporarily 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.

A Russian missile also hit the Dnipro hydroelectric station, Ukraine's largest dam, officials said.

Photos on social media showed a fire raging on the dam and the burnt-out carcass of a trolleybus.

Three people were killed in Zaporizhzhia, the city where the power station is located, Governor Ivan Fedorov said. He did not say whether it was in the strike on the dam.

The commander of Ukraine's ground forces Oleksandr Pavliuk also said Friday that Russia could be preparing to launch a summer offensive and was building a group of more than 100,000 troops.

In Russia, a woman was killed in a strike on Russia's Belgorod region, which borders Ukraine, the governor said.

And Moscow's FSB security service said it had arrested seven pro-Ukrainian partisans in the capital, the latest in a flurry of similar cases.