Russia on Monday said more than 60 soldiers were killed in a Ukrainian strike on Russian-controlled territory in a New Year assault, the biggest loss of life reported by Moscow so far.
Kyiv took responsibility for the strike, which it said took place in the occupied city of Makiivka in eastern Ukraine on New Year's Eve.
In a rare announcement that followed criticism by Russian military correspondents, the defence ministry in Moscow said 63 Russian servicemen were killed "as a result of a strike by four missiles" in Makiivka.
It was the biggest loss of life reported by the Russian defence ministry 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 ministry said US-supplied Himars rocket systems had been used and the target was a temporary deployment point.
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.
Earlier in the day, the strategic communications department of Ukraine's armed forces said nearly 400 Russian troops were killed in Makiivka.
The general staff, which usually communicates on such incidents, did not confirm the figure and said the human "losses" were still being established.
Russian war correspondents, who have gained influence in recent months, said hundreds could have been killed in the strike on a vocational school in Makiivka and accused Russia's top commanders of not learning from past mistakes.
Former Russian separatist leader Igor Strelkov said the troops, largely consisting of mobilised Russians, were stationed in an unprotected building that was "almost completely" destroyed because ammunition stored on the premises detonated in the strike.
"Ten months into the war, it is dangerous and criminal to consider the enemy a fool who does not see anything," said Andrei Medvedev, deputy speaker of the Moscow city legislature.
The Samara governor, Dmitry Azarov, said that among the victims were residents of his region, adding that a hotline was set up for their relatives.
On social media, some accused 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.
"Everything that the mobilised had on them remains under the rubble," said the announcement.
Another announcement said that residents in four of the region's cities -- Samara, Tolyatti, Syzran and Novokuibyshevsk -- will gather on Tuesday to mourn the troops.
Ukrainian President Volodymyr Zelensky said the country's forces had shot down more than 80 drones since the start of 2023.
"In the near future, this number may increase," he said.
He also said it was paramount to increase "the cost of a new mobilisation and the war on the whole for the terrorist state".
Russian strikes across Ukraine on New Year's Eve and New Year's Day killed at least five people and wounded dozens.
Kyiv 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.
On Monday evening, Kharkiv region head Oleg Synegubov said Ukraine's second-largest city and the surrounding region had once again been attacked with Russian missiles.
"Critical infrastructure has been targeted," he wrote.
General Valery Zaluzhny, commander in chief of the Ukrainian armed forces, said the army had so far liberated "40 percent of the territories occupied after February 24".
AFP journalists in Kramatorsk, the de facto administrative capital of Ukraine's eastern Donetsk region, reported four loud explosions on Monday evening.
The Russian Defense Ministry said on social media Monday night that it had conducted strikes near the city and in the Kharkiv region, claiming to have killed "more than 70 foreign mercenaries."
After suffering a series of humiliating battlefield defeats, Moscow began targeting 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 decried probable war crimes by Russian forces.
Ukraine and the European Union will hold a summit in Kyiv on February 3 to discuss financial and military support, Zelensky's office said in a statement on Monday that followed a phone call with European Commission President Ursula von der Leyen.
The leaders discussed the supply of "appropriate" weapons and a new 18 billion euro ($19 billion) financial assistance programme to Ukraine, with Zelensky pushing for the first tranche to be sent this month, according to the statement.
