"Condolences to the relatives of the dead and injured... This terrorist attack will not go unpunished," he wrote on Telegram.
He also added that United Russia will provide assistance to residents affected by the shelling of the city center by Ukrainians.
Earlier, the Russian Ministry of Defense said that the Armed Forces of Ukraine attacked Belgorod with two Vilkha missiles in cluster munition, as well as Czech-made Vampire MLRS rockets.
According to the latest data from the Russian Emergencies Ministry, 12 adults and two children were killed as a result of the shelling of Belgorod by Ukrainians. Another 108 people, including 15 children, were injured.