(Adds Russian defence ministry statement in paragraph 4)

       MOSCOW, Aug 23 (Reuters) - 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 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, hitting back as

Russian troops advance in eastern Ukraine.

    The Russian Defence Ministry said in a statement that its

air defence 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 spokeswoman 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 Aug. 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 km (18 miles) 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 modernisation

and extending the service life of equipment.    

(Reporting by Reuters; Writing by Mark Trevelyan and Anastasia

Teterevleva, Editing by Andrew Osborn and Timothy Heritage)

((moscow.newsroom@thomsonreuters.com;))