Moscow claimed on Sunday that its army conducted a "retaliatory strike" in a city in Donetsk that killed more than 600 Ukrainian servicemen. However, Ukrainian officials denied there were any casualties.

The Russian Defense Ministry said its missiles hit two temporary bases housing 1,300 Ukrainian troops in the city of Kramatorsk, killing more than 600 of them. Lieutenant-General Igor Konashenkov said the strikes were in retaliation for Ukraine's attack on Jan 1 in Makiivka, in which at least 89 Russian soldiers died.

But Ukraine's armed forces quickly rejected the Russian claim about the Kramatorsk strikes.

"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 Ukraine's Suspilne news. In the attack on Makiivka, Ukraine used United States-supplied HIMARS artilleries.

Cherevaty said Russia could not deliver high-precision strikes and the Russian statement did not say when exactly the strike had taken place.

Kramatorsk Mayor Oleksandr Honcharenko told Reuters that there had been no casualties.

Also on Sunday, the Ukrainian military claimed to have hit a residential hall of a medical university in Rubizhne, a town in the Lugansk region, killing 14 Russian soldiers housed there. The number of wounded was unknown, it said.

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

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

Also on Sunday, Russia and Ukraine swapped 50 captured soldiers each in a deal that both sides welcomed even as fighting continued between the troops.

Russia's Defense Ministry said the 50 returned Russian soldiers, "who were in mortal danger while in captivity", would be flown to Moscow for medical and psychological rehabilitation.

Prisoner swap

Andriy Yermak, head of the Ukrainian presidential office, confirmed the information and said Russia had freed 50 Ukrainian servicemen as part of the same deal.

"To be continued. We must bring all our people home, and we are on it," Yermak said on Twitter, posting images of the freed Ukrainian soldiers holding bags of food near a bus they were about to board and a video of them singing the Ukrainian national anthem once underway.

Separately, Russia and Belarus will hold joint air force exercises in Belarus from Jan 16 to Feb 1, the Belarusian Defense Ministry announced.

A Russian air division team arrived in Belarus on Sunday, while personnel, weapons, military and special equipment of the Russian armed forces will continue to arrive before the exercises.

Agencies via Xinhua contributed to this story.

renqi@chinadaily.com.cn