Russia on Monday admitted dozens of its soldiers were killed in a Ukrainian strike on Russian-controlled territory in a bloody New Year's weekend for both sides of the conflict.

In an extremely rare announcement, the Russian defence ministry said that 63 Russian servicemen were killed "as a result of a strike by four missiles" in the occupied city of Makiivka in eastern Ukraine.

It was the biggest loss of life reported by the Russian side so far in a conflict that has dragged on since President Vladimir Putin ordered Russian troops to invade on February 24 last year.

The Russian defence ministry did not say when the strike took place but Ukrainian forces are believed to have struck as Russian troops rang in the New Year.

The ministry said US-supplied Himars rocket systems had been used and the target was a temporary deployment point.

Without claiming the strike, Ukraine's military put the death toll much higher, claiming nearly 400 troops were killed.

Russian strikes in different parts of Ukraine on New Year's Eve and New Year's Day killed at least five people.

The Ukrainian capital again came under fire from Iranian-made drones on Monday, although Ukrainian forces claimed the majority were shot down by air defences.

Kyiv Mayor Vitali Klitschko reported an explosion in northeastern Kyiv and said emergency services were dispatched.

"An injured 19-year-old man was hospitalised in the Desnyanskyi district of the capital," he said.

Authorities later said he was hit by the falling debris.

Following the strikes, the power company Ukrenergo said the situation with the electricity supply in Kyiv was now "more complicated".

"That is why emergency shutdowns are now in effect," it said on social media.

Russia's New Year assaults -- which targeted downtown areas of large cities -- show a change in tactics, said an adviser to Ukrainian President Volodymyr Zelensky.

"Russia no longer has any military goals and is trying to kill as many civilians as possible and destroy more civilian facilities," Mykhailo Podolyak tweeted.

"A war to kill."

Hospital strike

On Saturday, Russian artillery hit the village of Naddniprianske outside the southern city of Kherson, severely wounding a 13-year-old boy.

Then the Russian army struck the hospital where the boy was lying in intensive care, smashing the windows.

"What did the 13-year-old boy do to these inhumans that they tried to kill him twice?" governor Yaroslav Yanushevych said on messaging app Telegram.

The Russian onslaught damaged the Kherson hospital and also left the city and the surrounding settlements without electricity.

Russian forces in November withdrew from Kherson, the only regional capital held by Moscow, but have continued to batter the city.

In Kyiv, police chief Andriy Nebitov at the weekend released a picture of the wreckage of a downed drone that featured the words "Happy New Year" in Russian.

"That is everything you need to know about the terror state and its army," he wrote.

'Extreme hardship'

After suffering a series of humiliating battlefield defeats, Moscow started to target electrical and other critical infrastructure in October.

The strikes have caused sweeping blackouts and cut off water supplies and heating to civilians as the temperature in some regions dropped below freezing.

The UN's human rights chief has warned the campaign has inflicted "extreme hardship" on Ukrainians, and also decried probable war crimes by Russian forces.

But Putin declared during his midnight address on New Year's Eve that "moral, historical rightness is on our side".

Moscow said its New Year's attacks had targeted the pro-Western country's drone production.

"The plans of the Kyiv regime to carry out terror attacks against Russia in the near future have been thwarted," Russia's defence ministry said.

Russia has accused Ukraine of targeting its domestic military sites and infrastructure.

In December, Moscow said it had shot down drones three separate times over or near Engels airfield, an airbase in southern Russia more than 600 kilometres (370 miles) from Ukraine. Falling debris killed three people in one of those attacks.

In early December, another base in Russia's Ryazan region also saw attacks from Ukraine's drones that killed three people, Moscow said.

On Monday, Russian officials said a Ukrainian drone had struck an energy facility in the southwestern Bryansk region neighbouring Ukraine.

Bryansk regional governor Alexander Bogomaz said on Telegram that the strike had cut off electricity to a village.