Ukraine says Russia is causing widespread blackouts in the Kharkiv and Donetsk regions.
On Saturday, Russia's Defense Ministry admitted it had pulled back troops in parts of Kharkiv.
They were overwhelmed by a Ukrainian counteroffensive that has made significant advances in the past week.
But on Sunday, Ukrainian President Volodymyr Zelenskyy blamed Moscow for the electricity cuts across eastern Ukraine.
He said Russia was deliberately targeting civilian infrastructure.
"A total blackout in the Kharkiv and Donetsk regions, a partial one in the Zaporizhzhia, Dnipropetrovsk and Sumy regions," Zelenskyy said in a statement on social media, blaming "Russian terrorists."
"No military facilities," he added. "The goal is to deprive people of light and heat."