In a rare disclosure, Russia's defense ministry said 63 soldiers were killed on New Year's Eve in a fiery blast that destroyed a temporary barracks in a vocational college in Makiivka, twin city of the Russian-occupied regional capital of Donetsk in eastern Ukraine.
Unverified footage posted online of the aftermath of the blast at the Russian barracks in Makiivka showed a huge building reduced to smoking rubble.
Ukraine and some Russian nationalist bloggers have put the Makiivka death toll in the hundreds, though pro-Russian officials say those estimates are exaggerated.
Russian military bloggers said the extent of the destruction at Makiivka was a result of storing ammunition in the same building as a barracks, despite commanders knowing it was within range of Ukrainian rockets.
Igor Girkin, a former commander of pro-Russian troops in eastern Ukraine who is now one of the highest-profile Russian nationalist military bloggers, said hundreds had been killed or wounded. Military equipment stored at the site was uncamouflaged, he said.
"What happened in Makiivka is horrible," wrote Archangel Spetznaz Z, a Russian military blogger with more than 700,000 followers on the Telegram messaging app.
"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?" he wrote. Commanders "couldn't care less", he said.
The fury in Russia extended to lawmakers.
Grigory Karasin, a member of the Russian Senate and former deputy foreign minister, not only demanded vengeance against Ukraine and its NATO supporters but also "an exacting internal analysis".
Sergei Mironov, a legislator and former chairman of the Senate, Russia's upper house, demanded criminal liability for the officials who had "allowed the concentration of military personnel in an unprotected building" and "all the higher authorities who did not provide the proper level of security".
Ukraine almost never publicly claims responsibility for attacks on Russian-controlled territory in Ukraine and Zelenskyy did not address the Makiivka strike in his nightly speech on Monday.
Rallies to commemorate the dead were held in several Russian cities, including Samara, where some came from, RIA Novosti news agency reported. Mourners laid flowers in the center of Samara.
"I haven't slept for three days, Samara hasn't slept. We are constantly in touch with the wives of our guys. It's very hard and scary. But we can't be broken. Grief unites ... We will not forgive, and, definitely, victory will be ours," RIA quoted Yekaterina Kolotovkina, a representative of a women's council at an army unit, as telling one of the rallies.
The strike on Makiivka came as Russia was launching what have become nightly waves of drone attacks on Kyiv and other Ukrainian cities. Ukrainian President Volodymyr Zelenskiy said in his nightly video address that the attacks were aimed at "exhausting our people, our anti-aircraft defenses, our energy."
Elsewhere, Ukraine's military General Staff said a Dec. 31 strike on a Russian-held area of the southern Kherson region had killed or injured some 500 Russian troops.
Reuters could not independently confirm the battlefield accounts.