President Volodymyr Zelensky says Moscow has bombed a boarding school in Ukrainian-occupied Russia where civilians were sheltering and preparing to evacuate.
The Ukrainian army said four people were killed and dozens - many of them elderly - were injured in the town of Sudzha in the Kursk region, which has been under Ukrainian control for five months.
More than 80 people are reported to have been rescued from the building.
The BBC has not been able to confirm Ukraine's claim that it was a deliberate Russian attack using a guided aerial bomb. Moscow blamed Ukraine for the bombing.