Russia's Defence Ministry said 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 smouldering rubble.
Earlier, Ukraine's defence 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 Defence 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 mobilised since September, many of them already sent to the front to bolster Russia's faltering military campaign in Ukraine.
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