"After yesterday's massive shelling, energy workers were able to unify the power system and reconnect three nuclear power plants to the power grid by morning," the ministry said on social media.

The plants should start supplying electricity by "this evening", it added.

The Russian strikes piled pressure on the Ukrainian grid, disrupting power supplies in southern and eastern regions, with water and electricity cuts in the capital Kyiv.

Energy providers in Kyiv were struggling Thursday to restore electricity with most of the capital still offline, mayor Vitali Klitschko said.

"Seventy percent of the capital remains without electricity," Klitschko said. "Energy companies are making every effort to return it as soon as possible," he added.

Water supplies were restored on Kyiv's left bank but only after engineers "worked all night," he said.

Klitschko said the city hoped to bring back water supplies on the right bank during the first half of the day.

Ukraine said Russian forces fired 70 cruise missiles at targets across the country on Wednesday, deepening an energy crisis after months of targeted attacks.

The governor of the eastern Kharkiv region, home to the country's second largest city, said Thursday the eponymous city was experiencing "issues with electricity supplies" and reported "emergency power shutdowns".

The head of the central region of Poltava, Dmytro Lunin, said authorities were "working around the clock to restore power."

"In the coming hours, we will start supplying energy to critical infrastructure and then to the majority of households," Lunin said.

About 50 percent of consumers in the central Dnipropetrovsk region had electricity, said the head of the Regional Military Administration, Valentyn Reznichenko.

"The energy supply situation is complicated. So shutdowns will continue in the region to reduce the pressure on the grid as much as possible," Reznichenko warned.

Repair work was ongoing elsewhere, including in the Rivne, Cherkasy, Kirovograd and Zhytomyr regions, officials said.