Ukraine’s national electricity operator imposed emergency power cuts on Wednesday after the country came under what the energy minister called a “massive” missile attack.

Air raid sirens wailed around 5:45 a.m. in the capital, Kiev, as much of the country was on alert for the missile launch. As people gathered in shelters at three o’clock, and the Air Force warned of the arrival of cruise missiles, Energy Minister Herman Galushchenko announced urgent steps.

“Due to the massive attack, the transmission system operator is applying preventive measures of restriction,” Mr. Galushchenko, they wrote in a statement on Facebook.

Emergency power cuts were implemented in six regions across the country, including Kharkiv in the northeast and Zaporizhzhia in the south, according to Ukrenergo, the national electricity operator.

Russian forces have repeatedly targeted Ukraine’s energy infrastructure in campaigns to blackout and deplete the country in winter.

The scale of Wednesday’s attack was not immediately clear. It came a day after Moscow threatened return because what was said was Ukraine’s use of Western-made long-range missiles for attacks inside Russia.