Air raid sirens wailed in Kyiv and across all Ukrainian regions on Sunday morning but no new Russian attacks were reported, officials said. The all-clear was later given.
Unconfirmed Ukrainian social media reports suggested the sirens may have been triggered after Russian jets took to the skies in Belarus and that the all-clear was sounded after the planes returned to their bases.
Reuters was unable to verify those reports.
Yuriy Ihnat, a spokesperson for Ukraine's air forces, told national television earlier on Sunday that Russian military jets were flying virtually around the clock.
"But we have increased readiness - everything that takes off must be under our control," Ihnat said.
Russia has carried out a series of missile and drone strikes on Ukrainian energy infrastructure since mid-October, knocking out power and causing emergency blackouts in many areas.
On Saturday a Russian strike on the southern city of Kherson, recently liberated by Ukrainian forces, killed at least 10 people, wounded 58 and left bloodied corpses on the road, authorities said, in what Kyiv condemned as wanton killing for pleasure. Moscow accused Ukraine of launching the attack.
