The Ukrainian attack on New Year's Day hit a building in the city of Makiivka, where Russian forces were stationed.

It is extremely rare for Moscow to confirm any battlefield casualties.

But this was such a deadly attack, says the BBC's Russia editor Steve Rosenberg, that staying silent probably wasn't an option.

It is the highest number of deaths acknowledged by Moscow in a single incident since the war began 10 months ago.

In a statement on Monday, Russia's defence ministry said Ukrainian forces fired six rockets using the US-made Himars rocket system at a building housing Russian troops. Two of them were shot down, it added.

Igor Girkin, a pro-Russian commentator, earlier said that hundreds had been killed and wounded, although the exact number was unknown because of the large number still missing.

The building itself was "almost completely destroyed", he said.

He added that the victims were mainly mobilised troops - that is, recent conscripts, rather than those who chose to fight. He also said ammunition was stored in the same building as the soldiers, making the damage worse.

"Almost all of the military equipment was also destroyed, which stood right next to the building without any disguise whatsoever," he wrote on Telegram.

Girkin is a well-known military blogger, who led Russian-backed separatists when they occupied of large parts of eastern Ukraine in 2014. He was recently found guilty of murder for his part in the shooting down of flight MH17.

Despite his hawkish stance, he regularly criticises the Russian military leadership and their tactics.

Several Russian lawmakers have also strongly criticised military commanders over the attack, saying commanders must be held to account for allowing troops to concentrate in an unprotected building within range of Ukrainian rockets, where ammunition may also have been stored.

Sergei Mironov - a former chairman of the Russian Senate - said it was obvious than neither intelligence nor air defence had worked properly.

According to the Ukrainian military's earlier statement, 300 were wounded in addition to the estimated 400 killed. Ukraine's army claims, almost daily, to have killed dozens, sometimes hundreds, of soldiers in attacks.

A later statement from the Ukrainian military's general staff said "up to 10 units of enemy military equipment" were "destroyed and damaged" in the strikes, and that "the losses of personnel of the occupiers are being specified".

Ukraine has not confirmed the strikes were carried out with Himars missiles, maintaining a long-held strategy of not releasing specific details about its attacks.