STORY: Speaking during his nightly video address, Zelenskiy said the death toll from the attack in the Donetsk town of Chasiv Yar was likely to rise.
"Two residential high-rise buildings were destroyed. Dozens of people are under the rubble. Six were saved. There are 15 names in the list of the dead and, unfortunately, this is not the final number," he said.
Donetsk Governor Pavlo Kyrylenko said the strike on the Chasiv Yar apartment building took place on Saturday evening.
The regional emergency service gave the death toll at 15 on Sunday afternoon, adding that 24 others could still be under the rubble.
Zelenskiy refuted claims that Russian air strikes had been "paused".
"34 airstrikes by Russian aviation in the past 24 hours, in one past day, is an answer to all those who invented this "pause"," he said, adding: "The Ukrainian army is holding on, holding firm...But, of course, a lot still needs to be done so that Russian losses become such that there will really be a pause."
Ukraine reported clashes with Russian troops on fronts in the east and south on Sunday, while Moscow said its forces struck Ukrainian army hangars storing U.S.-produced M777 howitzers, a type of artillery, near Kostyantynivka in Donetsk region.
Reuters could not independently verify battlefield accounts.