Moscow claimed on Sunday its army conducted a deadly "retaliatory strike" in the eastern Ukrainian city of Kramatorsk to avenge the deaths of 89 Russian troops killed in Makiivka.
"More than 600 Ukrainian servicemen were killed" as a result of a strike on Kyiv troops stationed in two buildings in Kramatorsk used as barracks, the Russian defense ministry claimed in a statement, calling the attacks a "retaliatory strike."
Ukraine's armed forces rejected the claim.
"This information is as true as the data that they have destroyed all of our HIMARS," Sergiy Cherevaty, spokesman for the Eastern group of the Ukrainian armed forces, told the Suspilne media outlet, referring to US-supplied rocket systems. He said Russia could not deliver high-precision strikes.
AFP could not independently verify the report.
On Saturday, AFP journalists in Kramatorsk, which is located in the eastern region of Donetsk, heard at least four explosions before midnight.
Both countries marked Orthodox Christmas on Saturday.
The Russian defense ministry did not say when exactly the strike had taken place but said that Russian intelligence had "over the past 24 hours" confirmed the points of temporary deployment of Ukraine's armed forces in Kramatorsk.
More than 1,300 Ukrainian troops were housed in two buildings, Moscow claimed.
Earlier Sunday, Pavlo Kyrylenko, the head of the Donetsk regional administration, said Russians launched seven rocket attacks on Kramatorsk.
He said that "an educational institution, an industrial facility and a garage cooperative" had been damaged and that there were no casualties.
In a New Year's attack, Ukraine struck a building in the occupied eastern Ukrainian town of Makiivka being used as a barracks.
Russia conceded 89 troops had died, in what was the worst single reported loss from a Ukrainian strike.
Russian President Vladimir Putin had ordered a 36-hour ceasefire to allow Orthodox Christians to mark Christmas which is celebrated on January 7 in Russia and Ukraine.
The unilateral ceasefire ended at 11:00 pm in Kyiv (2100 GMT) on Saturday.