Explosions shook the Ukrainian capital in early hours on Sunday, Kyiv Mayor Vitali Klitschko said, adding that information was being clarified and defense systems were downing air targets.

"Blasts in Kyiv," Klitschko said on the Telegram messaging app. "Information is being clarified. Details later. Anti-aircraft defense working in the region, there are downed targets."

Reuters' witnesses reported several blasts after air alerts were issued for Kyiv and a large swath of Ukraine.