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

Air-raid sirens wailed at around 5:45 a.m. in the capital, Kyiv, as most of the country was put on alert for missile launches. As people huddled in shelters into a third hour and the air force warned of incoming cruise missiles, the energy minister, Herman Galushchenko, announced emergency steps.

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

Emergency power shutdowns were applied 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 Ukrainian energy infrastructure in campaigns to black out, and wear down, the country in wintertime.

The scale of the assault on Wednesday was not immediately clear. It came a day after Moscow threatened to retaliate for what it said was Ukraine’s use of Western-made long-range rockets to strike inside Russia.