Russian forces have again targeted Ukraine's energy facilities, leaving part of Kyiv and other cities with no power and water.
A presidential aide said the situation across Ukraine was now critical.
Prosecutors say two people were killed in an attack on the capital. Plumes of smoke were seen billowing from around a power station near the Dnipro river.
Power and water were cut in Zhytomyr, west of Kyiv, and two facilities were badly damaged in Dnipro.
"Everyone should be ready, first, to save electricity, and second, rolling power blackouts are also possible if strikes continue," said Kyrylo Tymoshenko, the deputy head of the president's office.
"The entire population needs to prepare for a tough winter."
President Volodymyr Zelensky said on Twitter on Tuesday that in the past eight days, 30% of Ukraine's power stations had been destroyed, "causing massive blackouts across the country".
Ukrainian energy firm DTEK said two of its thermal power plants had been significantly damaged by Russian shelling, leaving one worker dead and six others wounded.
Russia has stepped up attacks in recent weeks on electricity infrastructure in cities away from the front lines. Officials have rushed to repair the damage, but the strikes, ahead of winter, have raised concerns about how the system will respond.