Work to restore electricity was also under way in seven other regions, said Kyrylo Tymoshenko, deputy head of Ukraine's presidential administration.
He wrote on the Telegram messaging app that emergency cut-offs were still possible, and national grid operator Ukrenergo said it was implementing planned outages across the entire country as repair work continued "around the clock."
"Please prepare for extended times of limited power supply: Stock up on water and charge your devices and power banks ahead of time to stay in touch with your close ones," it said in a statement.
Ukrainian officials said Tuesday's missile barrage was Russia's largest since its Feb. 24 full-scale invasion. Russian forces have increasingly targeted energy infrastructure in recent weeks, seriously damaging up to 40 percent of it, President Volodymyr Zelenskiy has said.
Oleksandr Kharchenko, director of the Energy Industry Research Center in Kyiv, said Tuesday's attacks had left about 12 million Ukrainians without power but that electricity had already been restored to the vast majority.
Kharchenko estimated that it would be five to six weeks before Ukraine's energy grid was repaired well enough for most customers to experience minimal or no cut-offs.
"But this is only assuming there are no further attacks," he told a briefing.
The onset of winter, and much colder temperatures, has increased concerns about the stability of heating supply but Tymoshenko said officials were not considering evacuating cities following the missile strikes.
(Additional reporting by Max Hunder; Editing by Timothy Heritage)
By Dan Peleschuk
