A concentrated, nine-minute-long Russian drone attack on Ukraine's second largest city of Kharkiv in the middle of the night killed six people and injured 64, including nine children, Ukrainian officials said on Wednesday.

The overnight attack followed Russia's two biggest air assaults of the war on Ukraine this week, part of intensified bombardments that Moscow says are retaliatory measures for Kyiv's recent attacks in Russia.

Elsewhere, two southern Ukrainian regions, Mykolaiv and Kherson, were left without electricity on Wednesday after Russian forces attacked an energy facility, the governors said.