The rare admission of the setback reflected the severity of the incident and of the domestic reaction to it, with influential pro-military figures voicing criticism of the Kremlin and suggesting that the death toll may have been much higher.
Kyiv also appeared to suggest that the number of soldiers killed was higher than Moscow's official numbers. NBC News was unable to verify claims from either side.
In a Telegram post Monday that followed hours of speculation, Russia's defense ministry said that sixty-three servicemen were dead after four HIMARS artillery warheads struck a “provisional base.” Two projectiles were shot down, it added.
Earlier, a senior Russian-backed official in occupied Donetsk said U.S.-supplied HIMARS had hit a “vocational school” in the city of Makiivka.
“A significant number of dead and wounded,” Daniil Bezsonov said Sunday night on Telegram.
While Ukraine has not taken responsibility for the attack, its armed forces posted a cryptic message on Telegram: “As a result of 'careless handling of heating devices,' neglecting security measures, and smoking in an unspecified place, Santa packed about 400 corpses of Russian servicemen in bags.”
"About 300 more have wounds of varying degrees of severity," the account added.
Ukrainian officials were not immediately available for comment.
On Sunday, the leader of the self-proclaimed Donetsk People’s Republic, Denis Pushilin, said many civilian facilities had come under Ukrainian fire over New Year’s Eve, according to the Russian news agency Tass.
Bezsonov criticized the “crowded deployment of personnel,” which he said helped lead to the casualties.
Russian forces have also been pummeling cities across Ukraine in recent days, launching missile and drone attacks on key civilian infrastructure.
Overnight, the capital, Kyiv, came under attack from the Iranian-made drones. Mayor Vitali Klitschko said 22 were destroyed over Kyiv, three in the outlying Kyiv region and 15 over neighboring provinces.
“As a result of the night shelling of the capital, energy infrastructure facilities were damaged. There are emergency power outages in the city,” Klitschko said Sunday night in a Telegram post.
Energy infrastructure facilities were damaged in the attack, and an explosion occurred in one city district, he said. It wasn’t immediately clear whether it was caused by drones or other munitions. A wounded 19-year-old man was hospitalized, Klitschko added, and emergency power outages were underway in the capital.
Since October, Russia has carried out airstrikes on power and water supplies almost weekly.