Russia said it killed 600 Ukrainian troops on Sunday in revenge for a Ukrainian assault on New Year's Eve that reportedly killed hundreds of Russians.Ukrainian soldiers fire artillery near the Donbass frontline in eastern Ukraine - Diego Herrera Carcedo/Anadolu Agency via Getty Images© Diego Herrera Carcedo/Anadolu Agency via Getty ImagesThe Russian defence ministry said it struck a building in Kramatorsk, in Eastern Ukraine, with a massed rocket attack.It said the building was being used as a temporary barracks and that it targeted two dormitories that were housing around 1,300 Ukrainian soldiers.

“Responding to a criminal strike executed by the Kyiv regime in the first moments of the new year against a temporary accommodation facility in Makiivka, a vengeance operation has been completed,” the Russian defence ministry said in a statement.

The statement referenced an attack that took place on New Year's Eve, in which Ukraine said it killed about 400 Russians after targeting a barracks 

If confirmed, the Russian attack is likely to be the most-deadly single attack on troops in the war so far.

However, some observers doubt Russia's claim since there have so far been no reports from the ground to support it. 

In addition, Ukraine took advantage of Russia's error in gathering hundreds of troops in one spot on New Year's Eve - making it a vulnerable target - and some believe Ukraine would be unlikely to make the same mistake themselves