A strike on a complex in the Russian-controlled Ukrainian city of Makiivka killed 63 Russian troops, the Russian defence ministry announced on Monday.
They did not say when the strike took place, but Ukrainian forces struck a college converted to a barracks using US-supplied Himars systems as newly-mobilised Russian troops rang in the New Year.
"As a result of a strike by four missiles with a high-explosive warhead on a temporary deployment point, 63 Russian servicemen were killed," the defence ministry said in a statement, a rare announcement of losses in Ukraine.
It is the biggest loss of life reported by the Russian side so far, though the real number of dead may be much higher.
Late on Monday, the General Staff of Ukraine's armed forces said in a statement that its forces were behind the strikes on Makiivka.
"Up to 10 units of enemy military equipment of various types were destroyed and damaged," the general staff said, adding that the human "losses" were still being established.
Earlier in the day, Ukraine's military said the real figure was around 400 Russian troops killed during the attack, which took place in the Moscow-controlled part of the eastern region of Donetsk.
Russian military bloggers agreed, saying several hundred troops could have died as a result of the strike.
Former Russian separatist leader Igor Strelkov said he received a report about the strike about 1 am on January 1 (2300 GMT on December 31).
He said "hundreds" have been killed and wounded, adding the Ukrainian army struck a facility that housed mobilised troops.
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On social media, some accused the Russian authorities of downplaying the death toll. "Dear God, who will believe in the figure of 63? The building has been completely destroyed," one Russian, Nina Vernykh, wrote on the country's largest social network, VKontakte.
An announcement on the social network urged Russians to collect clothes, medicines and equipment for those who survived the strike, AFP reports.
"Everything that the mobilised had on them remains under the rubble," said the announcement.
It was also a deadly weekend for Ukraine and Russian strikes in different parts of of the country 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."
But without a hit of irony, 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.
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