Kyiv Mayor Vitali Klitschko said at least 20 people were injured Tuesday in a Russian missile attack, part of a widespread wave of aerial assaults against Ukraine that also included strikes on the northeastern city of Kharkiv.

Klitschko said the falling debris from a downed missile caused a fire at a multi-story building in the Ukrainian capital.

He reported multiple explosions in the city as air defenses responded to the wave of Russian rockets, with fires sparked in at least five districts.

The Russian barrage, which came during the morning rush hour, also cut off electricity in parts of Kyiv.

In Kharkiv, Mayor Ihor Terekhov said the city was under a “major missile attack.”

Oleg Sinegubov, the head of the military administration in Kharkiv, said on Telegram that Russian strikes killed at least one person and injured 19 others.

Sinegubov said the attacks damaged residential and commercial buildings, in addition to infrastructure sites.

Ukraine’s air force said Tuesday that Russia also attacked overnight with 35 drones, but that Ukrainian air defenses downed all of them.

Tuesday’s attacks followed a warning by Russian President Vladimir Putin that Russia would intensify strikes on Ukraine following a deadly Ukrainian attack Saturday on the Russian city of Belgorod that killed 24 people and wounded more than 100 others.

During a visit at a military hospital Monday, Putin called Ukraine’s attack in Belgorod “a terrorist act,” accusing Ukrainian forces of targeting “right in the city center, where people were walking around, before New Year’s Eve” and alleged they had “purposefully hit the civilian population.”

He said Russia would continue to hit what he called “military installations” and added that he believed the “strategic initiative” in the drawn-out conflict in Ukraine was on the Russian side.

He claimed that Moscow wanted an end to the almost two-year long war but, he stressed, “only on our terms,” according to Russia’s state-run TASS news agency.

Some material for this report came from The Associated Press, Agence France-Presse and Reuters.