Successive waves of Russian missile strikes crippled almost half of Ukraine's energy system, Prime Minister Denys Shmyhal said on Friday, as heavy fighting raged in areas in the east and south.

With temperatures falling and the capital Kyiv seeing its first winter snow, authorities were working to restore power nationwide after some of the heaviest bombardment of Ukrainian civilian infrastructure in nine months of war.

The United Nations has warned of a humanitarian disaster in Ukraine this winter due to power and water shortages.

"Unfortunately Russia continues to carry out missile strikes on Ukraine's civilian and critical infrastructure. Almost half of our energy system is disabled," Shmyhal said.

He was speaking at a joint news conference with a vice-president of the European Commission, Valdis Dombrovskis, who offered Ukraine the 27-nation bloc's "unwavering support" and condemned Russia's "brutal war" on its neighbor.

Ukrainian President Volodymyr Zelenskyy said earlier that about 10 million people were without power in a country with a pre-war population of about 44 million. He said authorities in some areas ordered forced emergency blackouts.

"The aggressor country has officially recognized that its goal is to destroy our energy infrastructure and leave Ukrainians without electricity and heat," Ukraine's national grid operator Ukrenergo said on the Telegram messaging app.

It said Russia had launched six large-scale missile attacks on Ukraine's energy infrastructure from Oct. 10 to Nov. 15.

Russia's defense ministry said its forces had used long-range weapons on Thursday to strike defense and industrial facilities, including "missile manufacturing facilities."

Reuters was unable to verify battlefield reports.

Russian forces swept into Ukraine on Feb. 24 in what it said was a special military operation to eliminate dangerous nationalists. Kyiv calls Moscow's action an unprovoked imperialist land grab.