Ukraine says around 400 Russian soldiers have died in a missile attack on the occupied Donetsk region.
Russian officials contested the figure, admitting only to 63 of their troops being killed. Neither claim has been verified.
The attack hit a building in the city of Makiivka, where Russian forces were stationed.
It is extremely rare for battlefield casualties to be confirmed by Moscow.
But this was such a deadly attack, says the BBC’s Russian editor Steve Rosenberg, that staying silent most probably wasn’t an option.
In a statement on Monday, Russia’s defence ministry said Ukrainian forces fired six rockets using the US-made Himars rocket system at a building housing Russian troops. Two of them were shot down, it added.
Daniil Bezsonov, a senior Russian-backed official in the occupied parts of Donetsk, said the missile struck Makiivka two minutes after midnight on New Year’s Day.
Although access to Russian-controlled areas is restricted, a number of Russian commentators and bloggers acknowledged the attack – but said the numbers were lower than claimed by Ukraine.
But Igor Girkin, a pro-Russian commentator, said hundreds had been killed and wounded, although the exact number was still unknown because of the large number still missing.
The building itself was « almost completely destroyed », he said.
He added that the victims were mainly mobilised troops – that is, recent conscripts, rather than those who chose to fight. He also said ammunition was stored in the same building as the soldiers, making the damage worse.
« Almost all of the military equipment was also destroyed, which stood right next to the building without any disguise whatsoever, » he wrote on Telegram.
Girkin is a well-known military blogger, who led Russian-backed separatists when they occupied of large parts of eastern Ukraine in 2014. He was recently found guilty of murder for his part in the shooting down of flight MH17.
Despite his pro-Russian stance, he regularly criticises the Russian military leadership and their tactics.
According to the Ukrainian military, 300 were wounded in addition to the estimated 400 killed.
Ukraine’s army claims, almost daily, to have killed dozens, sometimes hundreds, of soldiers in attacks, so caution is needed. But if the claims are confirmed, this could be one of the deadliest attacks by Ukraine on Russian targets in the war.
Ukraine has not confirmed the strikes were carried out with Himars missiles, maintaining a long-held strategy of not releasing details about its attacks.
It merely said, sarcastically, that the deaths were the result of « careless handling of heating devices, neglect of safety measures, smoking in an unidentified place ».
Ukrainian authorities posted a picture of a Russian drone that says, in Russian, « Happy New Year »
Hours after the strike in Makiivka, Kyiv came under fire. A drone and missile attack targeted critical infrastructure, the Ukrainian capital’s regional governor Oleksiy Kuleba said.
One man in Kyiv was injured by debris from a destroyed Russian drone, the capital’s mayor added.
Mr Kuleba said the weapons were Iranian-made Shahed drones, adding that they were « targeting critical infrastructure facilities ».
« The main thing now is to stay calm and stay in shelters until the alarm is off, » he said.
All 39 Iranian made drones were eventually shot down by Ukraine, the military said. But Vitaly Klitschko, the mayor of Kyiv, said energy facilities were damaged, disrupting power and heating supplies.
Russia has been targeting Ukraine’s energy infrastructure for several months, destroying power stations and plunging millions into darkness during the country’s freezing winter.
