Ukraine's anti-corruption agency said on Monday it was conducting large-scale raids targeting the country's energy sector, an operation that comes after months of infighting over Kyiv's anti-graft efforts.

The National Anti-Corruption Bureau (NABU) said the operation – carried out in collaboration with the Specialized Anti-Corruption Prosecutor's Office – was designed "to expose corruption" in the key sector.

President Volodymyr Zelenskyy had tried this summer to strip powers from both agencies via a law removing their independence – a move that triggered rare public backlash and the largest protests since the start of the Russian invasion in February 2022.

Ukraine has long been plagued by corruption, which has been the focus of public protests throughout its 30-year independence.

Reforms were introduced after the revolution of 2014, but there are fears that Zelenskyy's abortive bid this summer to change the law governing the anti-graft agencies could undermine Ukraine's ambitions to join the European Union.

The president's office said in the summer that the law aimed to improve the functioning of the two bodies.

But it scrapped the legislation after public outcry and concerns from Kyiv's European partners.

NABU said a "high-level criminal organization" had "established a large-scale corruption scheme to influence strategic enterprises in the state sector," including major nuclear power provider Energoatom.

The investigation is said to involve 1,000 hours of audio recordings and took 15 months of work.

The bureau published photographs showing bags stashed full of dollar and euro banknotes but provided no further details about the operation.

It did not say where or when the photos were taken.

Russia has pounded Ukraine's strategic energy sector with missile and drone attacks throughout the nearly four-year invasion.

On Monday, authorities said the latest Russian attacks had left millions of people in Ukraine affected by power outages.

Ukrenergo, the state power company, announced hours-long scheduled blackouts across multiple regions as energy and heating sites are pummelled by Russia ahead of the winter months.

"Currently, in most regions, repair teams, energy suppliers and municipal services are working around the clock – everyone is involved," Zelenskyy said in a video message on Sunday evening.

"Restoration is ongoing, and although the situation is difficult, thousands of people are working to stabilize the system and repair the damage."

Ukrainian officials say Moscow is aiming to further destabilize the country by plunging the east into darkness and cold, making life so miserable for civilians that it triggers another wave of mass displacement.

Meanwhile, the former head of Ukraine's state-run electricity grid, Volodymyr Kudrytsky, was arrested last month on embezzlement charges.

Now on bail, he has denied the charges, which he says are politically motivated and "could not have happened without the involvement of the presidential office."

Ukrainian media reported that NABU had also raided the house of former Energy Minister German Galushchenko and close Zelenskyy ally Timur Mindich.

NABU did not comment on those reports in its statement on Monday.

Mindich is a co-owner of production studio Kvartal 95. It was founded in the early 2000s by Zelenskyy, who was a former star comedian before he ran for office.