Ukraine has labelled as "propaganda" a Russian claim that it killed hundreds of Ukrainian soldiers in an attack.
Moscow claimed, without providing any evidence, that a "mass missile strike" in the eastern city of Kramatorsk had killed more than 600 Ukrainian forces.
It said it was in retaliation for a Ukrainian attack on a Russian base that killed dozens of Russian soldiers on New Year's Day.
But the Ukrainian military says this is untrue.
"This is another piece of Russian propaganda," Serhiy Cherevaty, a spokesman for the Ukrainian army, told the BBC.
Russia's defence ministry said it had killed more than 600 Ukrainian servicemen in a strike on buildings temporarily housing Ukrainian forces. More than 1,300 Ukrainian troops were housed in two buildings, Moscow said.
It called the attacks a "retaliatory strike" to avenge the deaths of 89 Russian troops killed in Makiivka. Ukraine says as many as 400 people were killed or wounded in that incident, while numbers into the hundreds have been given by Russian nationalists on social media.
Moscow is yet to offer any proof of its claim about the Kramatorsk deaths.
By matching pictures of the attack published by local officials to Google satellite imagery and other images online, the BBC has confirmed the location of two sites about a mile apart in Kramatorsk.
The strikes happened near two school buildings – vocational schools number 28 and 47 – which match with the dormitory numbers provided by Russia. Moscow says the buildings were housing Ukrainian military personnel.
However, there's no visual evidence that shows these two buildings were badly hit or that there has been mass deaths on the scale claimed by Russia.
By James Waterhouse, BBC Ukraine correspondent
Not that Russia's 36-hour ceasefire remotely resembled a truce, but almost immediately after it ended, we felt seven or so explosions in the city of Kramatorsk.
The rattle of the windows made us decide to head to the shelter. Until then, we'd only heard faint thuds and a few sirens. This marked a return to the almost daily missile strikes which Kramatorsk has had to endure.
Moscow's truce had only led to the faintest of let ups in the city, which can't be said for the surrounding area.
The local mayor posted images of damaged buildings and said two educational sites, eight apartment blocks and garages were damaged from the explosions.
There were no reported casualties, contrary to the claims from the Kremlin.
There was further shelling in various parts of Ukraine overnight after the end of what Russian President Vladimir Putin said was a 36-hour ceasefire by Russian forces so Orthodox Christians could celebrate Christmas. Evidence suggests this so-called ceasefire was not adhered to by Moscow.
Ukrainian officials said at least one person was killed in the Kharkiv region in the north-east.
Explosions were also reported in the southern cities of Zaporizhzhia and Melitopol.
Separately, Russia's defence ministry said that Ukraine had returned 50 captured Russian soldiers after negotiations. Kyiv confirmed that it had received the same number of soldiers in return from Russia.
Additional reporting by Jake Horton and Hanna Chornous
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