The Russian military claimed to have carried out a deadly missile strikes on barracks used by Ukrainian troops in retaliation for the deaths of dozens of Russian soldiers in a rocket attack a week ago.
Ukrainian officials denied there were any casualties.
The Russian Defence Ministry said on Sunday its missiles hit two temporary bases housing 1300 Ukrainian troops in Kramatorsk, in the eastern Donetsk region, killing 600 of them. Ministry spokesman Igor Konashenkov said the strikes were retaliation for Ukraine's attack in Makiivka, in which at least 89 Russian soldiers died.
Serhii Cherevatyi, a spokesperson for Ukraine's forces in the east, told The Associated Press that Russian strikes on Kramatorsk damaged only civilian infrastructure, adding: "The armed forces of Ukraine weren't affected."
The Donetsk regional administration said seven Russian missiles hit Kramatorsk and two more hit Konstantynivka, without causing any casualties. It said an educational institution, an industrial facility and garages were damaged in Kramatorsk, and an industrial zone was hit in Kostyantynivka.
In the early hours of January 1, Ukrainian forces fired rockets at a facility in Makiivka, also in the eastern Donetsk region, where Russian soldiers were stationed, killing dozens of them in one of the deadliest attacks on the Kremlin's forces since the war began more than 10 months ago.
Also on Sunday, the Ukrainian military claimed to have hit a residential hall of a medical university in Rubizhne, a town in the Russian-occupied eastern Luhansk region, killing 14 Russian soldiers housed there. The number of wounded was unknown, it said.
Elsewhere in the east, Donetsk Governor Pavlo Kyrylenko said one person was killed in strikes on Bakhmut, and eight others were wounded.
In the northeastern Kharkiv region, the town of Merefa was hit during the night, killing one person, and two other settlements in the region were shelled, Governor Oleh Syniehubov said.
The latest developments come after Russian forces ended a partially observed cease-fire timed to coincide with Orthodox Christmas celebrations on Saturday.