The blackouts have reportedly affected around nine million people in eastern regions including Kharkiv and Donetsk.

It comes after Ukraine said it has retaken over 3,000 sq km during a rapid counter-offensive in the east, the BBC said, the figures though are not verified.

Kharkiv mayor Ihor Terekhov said Russian attacks on civilian infrastructure left much of his city without power or water. He called it a vile and cynical attempt at revenge for the Ukrainian army's recent successes.

Two missile strikes were heard late evening, the BBC report said.

The governor of the neighbouring Sumy region said over 130 settlements in one district alone were without power.

Similar problems have been reported in Dnipropetrovsk and Poltava regions, the BBC said.

In a defiant post on social media after the power cuts, Zelensky accused Russia of carrying out "terrorist acts" by targeting civilian infrastructure.

"Cold, hunger, darkness and thirst are not as terrible and deadly for us as your 'friendship and brotherhood'," he wrote on Telegram.

Ukraine has claimed it has tripled territorial gains over 48 hours.

President Zelensky said 1,000 sq km had been retaken on Thursday evening. That figure rose to 2,000 sq km on Saturday evening, then to 3,000 on Sunday.

Although the media has no access to front lines but several videos on social media show Ukrainian troops present in towns and villages that were until recently held by Russia.

(With UNI inputs)