"Attacks on civilians and civilian infrastructure violate international humanitarian law, are unacceptable and must stop immediately," RIA Novosti quoted the office as saying.
The U.N. added that the strikes "should of course be condemned."
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 Russian Emergencies Ministry, as of 17:58 Moscow time, 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.