Air raid sirens wailed across Ukraine on Tuesday after warnings by the country's leaders that Russia could launch a new wave of missile and drone strikes, but there were no immediate reports of attacks.
Russia has carried out several waves of attacks on Ukrainian energy infrastructure since October, causing power outages across the country.
Several minutes after the first air raid alerts were issued on Tuesday, there had been no reports of missiles being fired at Ukraine.
Ukrainian media said the alerts may have been triggered by MiG fighter jets that took off from Ryazan, near Russia's border with Ukraine, and flew towards Belarus.