Russia's Defense Ministry said on Monday that 63 Russian soldiers had been killed in a Ukrainian New Year's Eve attack on their quarters in the Russian-controlled part of Ukraine's Donetsk province.
Footage posted online showed a building purported to be a vocational college in the town of Makiivka, twin city of the regional capital Donetsk, reduced to a field of smoldering rubble.
Earlier, Ukraine's defense ministry said as many as 400 Russians had been killed.
Daniil Bezsonov, a senior Russian-backed regional official in the Moscow-controlled parts of the Donetsk region, said the vocational college had been hit by U.S.-made HIMARS rockets at around midnight, as people in the region would have been celebrating the start of the New Year.
One Russian pro-war military blogger known as Rybar, who has more than one million subscribers on the Telegram messaging app, said more than 100 people had been wounded in the strike and that rubble was still being cleared.
Rybar said there had been about 600 people in the building, and that ammunition had been stored in the same facility.
Russia's Defense Ministry gave its estimate of the death toll only in the final paragraph of a 528-word daily battlefield roundup.
Reuters was unable to verify the battlefield accounts, but was able to confirm the location of the video from the nearby buildings and road layout seen in the footage, although not the date that it was filmed.
A source close to the Russian-installed Donetsk leadership told Reuters the building had been used to house some of the 300,000 or more soldiers mobilized since September, many of them already sent to the front to bolster Russia's faltering military campaign in Ukraine.
Russian nationalist war bloggers seethed with anger at the heavy loss of soldiers.
Igor Girkin, a nationalist and former Federal Security Service (FSB) officer who helped Russia annex Crimea in 2014 and then organize pro-Russian militias in eastern Ukraine, said in a Telegram post at 0908 GMT on Monday that "the number of dead and wounded runs into many hundreds".
Girkin, who has bitterly criticized Russia's military failures in Ukraine, also said ammunition had been stored at the facility.
"This is not the only such (extremely dense) deployment of personnel and equipment in the destruction zone of HIMARS missiles," he wrote. "And - yes - this is not the first such case."
Archangel Spetznaz Z, another Russian military blogger with more than 700,000 followers on Telegram, wrote:
"Who came up with the idea to place personnel in large numbers in one building, where even a fool understands that even if they hit with artillery, there will be many wounded or dead?"