Ukrainian nuclear firm Energoatom has carried out pre-winter maintenance on three of nine nuclear reactors and work is due to start on another in the coming days, it said on Thursday.

Ukraine now relies mainly on three nuclear power plants for electricity after Russian missile attacks that have destroyed most of its thermal power plants and at least two hydro stations.

Its fourth plant, Zaporizhzhia, was occupied by Russian forces in the first weeks of Moscow's 2022 invasion of the country.

State-run Energoatom said maintenance is being carried out on power units at the Rivne, South Ukraine and Khmelnytskyi power plants.

Recent Russian missile strikes have caused power outages in many regions and forced Kyiv to begin large-scale imports of electricity from the European Union.

Power grid operator Ukrenergo said imports could total 27,671 Mwh on Thursday, slightly less than the record high 29,796 Mwh imported on Wednesday.

Ukrenergo said it would continue to restrict power supplies to consumers on Thursday evening.

Earlier this month, power limits were more significant, but planned maintenance to a nuclear unit could bring back the long blackouts.

"In the next days, Energoatom will take out another 1,000 MW unit for repair. Taking this into account, as well as considering the increase in electricity consumption in July, the generation deficit in the power system may increase," Energoatom said.

The company says that scheduled work this summer taking advantage of longer daylight hours will make it possible to reduce the duration of forced power cuts in the autumn.