Russia has fired dozens of missiles and drones at Ukraine, killing 15 people, according to Ukrainian officials.
The two countries also traded blame for a strike on a boarding school sheltering civilians in the Ukrainian-occupied town of Sudzha in Moscow's Kursk region, where Kyiv launched a major cross-border assault last August.
The Ukrainian military said four people were killed in the attack, with dozens more rescued from the rubble.
Russia has not given a toll.
Fighting in the nearly three-year war has shown no signs of de-escalating, despite US President Donald Trump's promise to enact a ceasefire within "24 hours" of taking office on January 20.
At least 15 people were killed in Russian strikes on central and eastern Ukraine overnight Friday to Saturday, according to regional authorities and police.
Eleven of those, including a child, were killed by a missile that hit a residential building in the central city of Poltava, the local administration said.
Firefighters could be seen searching through the smouldering ruins of a building in AFP images from the scene.
"God saved us," said Olena Svyryd, a resident of a neighbouring building.
"Opposite us, on the fifth floor, a woman, my friend, was taken out. No, she's not alive. She was crushed by the wall. There were a lot of casualties."
Kateryna Yamshchykova, acting mayor of Poltava, said rescue operations were ongoing.
"Doctors in the hospital are fighting for our wounded," she said.
Finger pointing after boarding school strike
Ukraine accused Russia on Saturday of killing four people in a strike on a boarding school sheltering civilians in the Kursk region town of Sudzha, which Kyiv has occupied for over five months.
Moscow responded on Sunday by accusing Kyiv's forces of launching the attack.
"On February 1, the Ukrainian Armed Forces committed another war crime by launching a targeted missile strike on a boarding school in the city of Sudzha," said a statement from Russia's defence ministry.
The defence ministry did not mention any deaths, while Kursk region's active governor Aleksandr Khinshtein said "there is no reliable information about the number of victims yet".
Kyiv launched a surprise cross-border offensive into the Kursk region last August, seizing dozens of villages and small towns including the regional hub of Sudzha — home to about 6,000 people before the fighting.
"Russian aviation struck a boarding school in the town of Sudzha, Kursk region, with a guided aerial bomb," the Ukrainian military's general staff said on Telegram.
"The strike was carried out on purpose."
It said "dozens of local residents were inside the building preparing to evacuate" at the time of the attack, and that rescue work was under way.
"In the course of the rubble removal works, 84 civilians were rescued and provided with medical aid, their health condition is satisfactory, four are in serious condition, and four people died," it said in a later post.
Ukrainian President Volodymyr Zelenskyy called Russia "devoid of civility", sharing a video on social media showing a heavily damaged building, as well as a wounded man lying on the ground.
"They destroyed the building even though dozens of civilians were there," Mr Zelenskyy said in a post on X.
"Russian bombs destroy Ukrainian homes the same way. And even against their own civilians, the Russian army uses similar tactics."
A Russian official in Kursk told AFP last week that authorities were working "constantly" to secure the return of Russian civilians caught behind the front lines.
Thousands of Russian civilians are thought to be trapped by fighting in the border region.
Russia says it 'liberated' village
Moscow has been advancing on the battlefield for more than a year, and its invasion of Ukraine will this month hit the three-year mark.
The Russian military said on Saturday its troops had "liberated" the village of Krymske in the north-eastern suburbs of the city of Toretsk.
Toretsk in the eastern Donetsk region has been in the Kremlin's sights for months, as its capture would enable Russia to obstruct vital Ukrainian supply routes.
Both Mr Trump and his Russian counterpart Vladimir Putin have said they are ready for talks on ending the war, but neither side has said when or how.
Mr Trump has been critical of the billions Washington has spent arming Ukraine, while threatening to impose additional sanctions on Russia if Mr Putin does not reach a "deal" to end the war.
Mr Putin said last month he was willing to hold talks with Ukraine, but not with Mr Zelenskyy, whom he called "illegitimate".
ABC/wires
