The Ukrainian defence ministry did not immediately respond to a request for comment on the allegation, the second that Moscow has made in two days.

The nuclear plant is located in the Kursk region of western Russia, where fierce fighting has raged since Ukrainian forces launched a surprise incursion on Aug 6.

Russian state news agency TASS quoted an unnamed source as saying the drone was shot down near a storage facility for spent nuclear fuel. Reuters could not independently confirm details of the alleged incident.

TASS quoted Russia’s foreign ministry spokesman Maria Zakharova as saying it was an “act of nuclear terrorism” that required an immediate response from the UN watchdog, the International Atomic Energy Agency.

The head of the IAEA, Rafael Grossi, is due to visit the power station next week. He has appealed for maximum restraint to avoid a nuclear accident.

Russia’s president Vladimir Putin first accused Ukraine on Thursday of trying to attack the facility, and said Moscow had informed the IAEA. He did not provide details or evidence of a Ukrainian attack.

Ukraine’s lightning Aug 6 incursion into Kursk, the biggest into Russia by a foreign power since World War Two, saw thousands of Ukrainian troops punch through Russia’s western border, catching Moscow by surprise.

Fierce fighting around 30km from the nuclear plant has raged since then as Russian troops battle to dislodge the Ukrainian soldiers who have sought to consolidate and expand the territory they control.

The plant has four reactors, of which two are operational. Construction of two more reactors began in 2018.

Russian state nuclear firm Rosenergoatom said on Friday that unit No 4 at the plant would be disconnected from the grid on Sunday for what it called “scheduled preventive maintenance” lasting 59 days. It said the work would involve modernisation and extending the service life of equipment.