Three people have now been confirmed dead following a Russian drone attack on the northeastern Ukrainian city of Sumy Monday, the city's mayor said Tuesday.

Four Russian drones struck the center of the city on Monday, hitting two apartment blocks and an administrative building, according to local officials. 

The number of people wounded has also increased, from 19 to 21, Mayor Oleksandr Lysenko said.

The Sumy regional military administration called the strike "another terrorist act of the Russian Federation" and declared Tuesday a day of mourning in the city.