Wednesday’s series of drone and missile attacks damaged homes, vehicles and power lines in multiple districts of Belgorod, Gladkov wrote on Telegram.
He shared photographs from Maslova Pristan showing a sports centre with its roof and walls blown apart, and said rescue workers were clearing debris and assessing the damage.
In the nearby town of Moshchenoye, six people – including a child – were injured when a rocket and drone struck a truck, while a man was hurt when a car was hit in Masychevo.
The strikes follow two days of similar attacks that have caused blackouts and structural damage in Belgorod city and surrounding areas. Videos verified by BBC Verify showed explosions and fires near a local power station late on Tuesday.
Nearly 40,000 residents were affected by power cuts across seven municipalities, according to Gladkov.