Russia launched an air attack on Kyiv, with air defense systems engaged on the outskirts of the city in repelling the strikes, the mayor of the Ukrainian capital and military administration officials said early Sunday.

"Air defense units operating, air raid alert continues," Kyiv Mayor Vitali Klitschko wrote on the Telegram messaging app.

Reuters witnesses said they heard at least two explosions in what sounded like air defense units at work.

It was not immediately clear if the attack caused any damage or injuries.

Serhiy Popko, head of Kyiv's military administration, said on Telegram that the capital remained under the threat of Russia's ballistic missiles.

Kyiv, its surrounding region and all of eastern Ukraine were under air raid alerts, Ukraine's air force said on Telegram.