(CNN) — An apparent Ukrainian strike in Russian-occupied eastern Ukraine appears to have killed a large number of Russian troops, according to the Ukrainian military, pro-Russian military bloggers and former officials.

The strike took place just after midnight on Sunday, New Year’s Day, on a vocational school housing Russian conscripts in Makiivka, in the Donetsk region, according to both Ukrainian and pro-Russian accounts.

The attack has led to vocal criticism of Moscow’s military from pro-Russian military bloggers, who claimed that the troops lacked protection and were reportedly being quartered next to a large cache of ammunition, which is said to have exploded when Ukrainian HIMARS rockets hit the school.

The Ukrainian military said later on Monday that the number of Russian servicemen killed in Makiivka is “being clarified” after claiming earlier that around 400 Russian soldiers were killed and a further 300 were wounded. It has not directly acknowledged a role in the strike. CNN cannot independently confirm those numbers or the weapons used in the attack.

Some pro-Russian military bloggers have also estimated that the number of dead and wounded could run in the hundreds.

The Russian defense ministry on Monday acknowledged the attack and claimed that 63 Russian servicemen died, which would make it one of the deadliest single episodes of the war for Moscow’s forces.

Russian senator Grigory Karasin said that those responsible for the killing of Russian servicemen in Makiivka must be found, Russian state news agency TASS reported Monday.

Karasin, who is chairman of the International Affairs Committee of the Federation Council, said the deaths should not be forgotten.

Video reportedly from the scene of the attack circulated widely on Telegram, including on an official Ukrainian military channel. It shows a pile of smoking rubble, in which almost no part of the building appears to be standing.

“Greetings and congratulations” to the separatists and conscripts who “were brought to the occupied Makiivka and crammed into the building of vocational school,” the Strategic Communications Directorate of the Chief Commander-in-Chief of the Armed Forces of Ukraine said on Telegram. “Santa packed around 400 corpses of in bags.”

The Russian defense ministry said the Ukrainian attack used US-made HIMARS rockets.

Daniil Bezsonov, a former official in the Russia-backed Donetsk administration, said on Telegram that “apparently, the high command is still unaware of the capabilities of this weapon.”

“I hope that those responsible for the decision to use this facility will be reprimanded,” Bezsonov said. “There are enough abandoned facilities in Donbas with sturdy buildings and basements where personnel can be quartered.”

A Russian propagandist who blogs about the war effort on Telegram, Igor Girkin, claimed that the building was almost completely destroyed by the secondary detonation of ammunition stores.

“Nearly all the military equipment, which stood close to the building without the slightest sign of camouflage, was also destroyed,” Girkin said. “There are still no final figures on the number of casualties, as many people are still missing.”

Girkin has long decried Russian generals whom he claims direct the war effort far from the frontline, calling them “unlearned in principle” and unwilling to listen to warnings about putting equipment and personnel so close together in HIMARS range. Girkin was previously minister of defense of the self-proclaimed, Russian-backed Donetsk People’s Republic, and was found guilty by a Dutch court of mass murder for his involvement in the downing of Malaysia Airlines Flight 17 over eastern Ukraine in 2014.

Sergey Markov, another pro-Russian military blogger, said there was “a great deal of sloppiness” on the part of the Russian command.

Boris Rozhin, who also blogs about the war effort under the nickname Colonelcassad, said that “incompetence and an inability to grasp the experience of war continue to be a serious problem.”

“As you can see, despite several months of war, some conclusions are not made, hence the unnecessary losses, which, if the elementary precautions relating to the dispersal and concealment of personnel were taken, might have not happened.”

Donetsk has been held by pro-Russian separatists since 2014 and it is one of four Ukrainian regions that Moscow sought to annex in October in violation of international law.

The news comes after the Ukrainian military claimed that more than 700 Russian soldiers had been killed Saturday, but did not specify where.

Russian forces “lost 760 people killed just yesterday, (and) continue to attempt offensive actions on Bakhmut,” the military’s general staff said Sunday.

Russian units have been pressing an offensive towards the city of Bakhmut in Donetsk for months but have suffered heavy losses as Ukrainian forces have targeted them in what is largely open rural territory.

Air raid sirens sounded across Ukraine over the weekend as fresh rounds of Russian missile strikes hit several regions. The attacks killed at least six people in the Donetsk, Kharkiv and Chernihiv regions, while a man was injured early Monday.

Ukraine claims hundreds of Russian troops killed in strike; Moscow says 63 died