PARIS (REUTERS) - Unidentified drones illegally overflew five French nuclear power plants overnight, a source with knowledge of the matter said on Saturday, in the latest of a series of unexplained incidents that have raised safety concerns.

The small unmanned aircraft were detected late on Friday above the five plants in northern and eastern France, the source said, confirming an earlier report by Agence France-Presse.

State-controlled utility EDF, which operates the plants at Penly, Flamanville, Saint-Laurent-des-Eaux, Dampierre and Fessenheim, could not confirm the latest drone incursions, a spokeswoman said. French law bans aircraft of all types from flying within five kilometres of nuclear facilities.

Interior Minister Bernard Cazeneuve said on Thursday that France had launched an investigation after drones were sighted over seven EDF nuclear plants between Oct. 5 and Oct. 20.

The power company has insisted that the mystery flights pose no safety or security threat. But the incidents have fueled public anxiety in France, the world's most nuclear-dependent country with 58 reactors on 19 sites.

Greenpeace protesters forced their way into the Fessenheim facility earlier this year and have a history of breaking into French nuclear plants. The environmental campaign group has denied any involvement in the unmanned flights.