Russia accused Ukraine on Friday of trying to attack the Kursk nuclear power station overnight in what it called an act of “nuclear terrorism,” days before the head of the U.N. atomic watchdog is due to visit the site.

The Ukrainian defense 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, hitting back as Russian troops advance in eastern Ukraine.

The Russian Defense Ministry said in a statement that its air defense units had shot down three Ukrainian drones overnight in the Kursk region and spoke of thwarting a Ukrainian attempt to carry out “a terrorist attack” against Russian facilities.

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

TASS quoted Russian Foreign Ministry spokesperson Maria Zakharova as saying it was an “act of nuclear terrorism” that required an immediate response from the U.N. 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.

Russian President Vladimir Putin accused Ukraine on Thursday of trying to attack the facility, and said Moscow had informed the IAEA. He did provided no details or evidence of a Ukrainian attack.

Ukraine’s August 6 incursion into Kursk, in which thousands of Ukrainian troops punched through Russia’s border, is the biggest into Russia by a foreign power since World War Two and Moscow was caught by surprise.

Kyiv has said it has carved out a buffer zone from an area that Russia, which sent tens of thousands of troops into Ukraine in 2022, has used to pound targets in Ukraine.

Fighting around 30 kilometers 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 number 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 modernization and extending the service life of equipment.