Russia said Sunday it had carried out a devastating "retaliatory strike" in eastern Ukraine to avenge a recent deadly attack on its troops, a claim quickly denied by Kyiv.

"More than 600 Ukrainian servicemen were killed" by a missile strike on troops stationed in two buildings used as barracks in Kramatorsk, said a Russian defence ministry statement.

Russia described the attack as a "retaliatory strike" following Ukraine's New Year's Eve missile salvo in the eastern Ukrainian town of Makiivka, which Moscow said killed 89 of its soldiers.

Ukraine's armed forces rejected Russia's claim about the Kramatorsk attack.

"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. In the attack on Makiivka, Ukraine used US-supplied HIMARS missiles.

Cherevaty said Russia could not deliver high-precision strikes.

The Russian statement did not say when exactly the strike had taken place -- only that Russian intelligence had "over the past 24 hours" located more than 1,300 Ukrainian troops in two buildings in Kramatorsk.

The industrial city, in the eastern Donetsk region, has been the target of repeated Russian missile attacks.

- Four blasts before midnight -
Earlier Sunday, Pavlo Kyrylenko, the head of the Donetsk regional administration, said Russian forces had launched seven rocket attacks on Kramatorsk "after midnight".

He said "an educational institution, an industrial facility and a garage cooperative" had been damaged and that there had been no casualties.

AFP could not independently verify the situation. On Saturday, the Orthodox Christmas, AFP journalists in Kramatorsk heard at least four explosions before midnight.

In Ukraine's attack, its missiles struck a building in Makiivka being used as a barracks.

While Russia conceded 89 troops had died, the worst single reported loss from a Ukrainian strike since the war began last February, one statement from Ukraine's military claimed the losses came to more than 400.

Russian President Vladimir Putin ordered a 36-hour ceasefire to allow Orthodox Christians to mark Christmas, which was celebrated Saturday in Russia and Ukraine.

Ukrainian officials, who dismissed the ceasefire as a cynical propaganda move, said Russia had in any case not respected it.

In his evening address on Sunday, Ukrainian President Volodymyr Zelensky denounced "Russian shelling of Kherson with incendiary ammunition right after Christmas", in the southeast.

"The strikes on Kramatorsk and other cities of Donbas targeted civilian objects, and precisely when Moscow was reporting on the alleged 'silence' of the army," he added.

Putin's unilateral ceasefire ended at 2100 GMT Saturday: 11:00 pm in Kyiv, but midnight in Moscow.