The Sumy region borders Russia's Kursk region and has been regularly shelled by Russian forces for months.
"Russians dropped a bomb on a residential building. Two children and one adult were injured. One entrance of the apartment building was destroyed," Sumy's military administration said on Telegram.
A rescue operation was under way to find people who may be trapped by rubble, officials said.
Russia, which began its full-scale invasion of Ukraine in February 2022, did not immediately comment on the events in Sumy.
Meanwhile, operations at an airport in the Russian city of St Petersburg were reinstated on Saturday evening after they were temporarily suspended for safety reasons.
In the morning, the Rosaviatsiya aviation authority said no take-offs and landings would be permitted for the time being.
The Russian defence ministry reported that 16 Ukrainian drones were destroyed overnight over the regions of Bryansk, Smolensk, Belgorod, Pskov and Leningrad - the area surrounding St Petersburg.
There have been repeated restrictions at airports in Russia in connection with Ukrainian drone attacks.
Due to the deployment of Russian air defence, take-offs and landings are sometimes not possible.
According to the Ukrainian air force, Ukraine repelled 81 drone attacks by Russia during the night.
Of these, 34 drones were shot down and the location of 47 was lost, mostly due to the use of electronic defence systems.
The drones that were shot down damaged private homes in the Chernihiv and Sumy regions.
with DPA
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