Air raid sirens sounded across all of Ukraine's regions on Wednesday but there were no immediate reports of any new Russian missile strikes.
The capital Kyiv and other regions sounded the all-clear almost two hours later.
Ukrainian social media reports said the nationwide alert may have been declared after Russian jets stationed in Belarus took off.
Reuters was unable to immediately verify that information.
Russian forces stepped up mortar and artillery attacks on the recently liberated city of Kherson in southern Ukraine on Wednesday, Ukraine's military said, while also exerting constant pressure along front lines in eastern regions of the country.
Russia fired 33 missiles from multiple rocket launchers at civilian targets in Kherson in the 24 hours leading up to early Wednesday, the General Staff of Ukraine's Armed Forces said in its morning report. Russia denies targeting civilians.
Heavy fighting also persisted around the Ukrainian-held city of Bakhmut, now largely in ruins, in the eastern province of Donetsk, and to its north, around the cities of Svatove and Kreminna in Luhansk province, where Ukrainian forces are trying to break Russian defensive lines.
The current state of the war
Britain's defense ministry said in its latest update on the military situation in Ukraine that Russia had likely reinforced the Kreminna section of the frontline as it is logistically important to Moscow and has become relatively vulnerable following recent Ukrainian advances further west.
There is still no prospect of talks to end the war, now in its 11th month.
Ukrainian President Volodymyr Zelensky is vigorously pushing a 10-point peace plan that envisages Russia fully respecting Ukraine's territorial integrity and pulling out all its troops.