Nearly half of Ukraine's electricity grid remains damaged, a private operator said on Thursday, a week after the latest Russian strikes on the country's energy infrastructure disrupted power to millions of people.

"Russia has destroyed 40 percent of the Ukrainian energy system with terrorist missile attacks. Dozens of energy workers were killed and wounded," DTEK company said in a statement on social media.

After suffering humiliating military defeats on the ground, Russia began targeting Ukrainian energy facilities in October, causing severe damage and power shortages.

Last week, a latest series of massive strikes on these sites left entire regions across Ukraine cold and dark.

"Electrical engineers are doing everything possible and impossible to stabilise the situation regarding energy supply," the company said, saying its technical teams are working "day and night" to quickly repair the infrastructure.

Authorities said Wednesday that nine people had died in fire-related accidents in the country over the past 24 hours, as Ukrainians are forced to find alternate heating sources.

Britain's defence ministry said in a note Thursday that continuing Russian strikes on energy infrastructure result in "indiscriminate, widespread humanitarian suffering across Ukraine".

"However, its effectiveness as a strategy has likely been blunted because Russia has already expended a large proportion of its suitable missiles