Another unidentified drone was spotted this week flying over a French nuclear plant, adding to a spate of mystery drones over the past month, the nuclear plant at Bugey reported on its website Saturday.

"The surveillance teams at the Bugey plant detected the illegal presence of an aircraft similar to a drone in the air perimeter of the plant," a statement on the website said, indicating it happened on Thursday.

The Bugey plant located in east-central France, which has seen four mystery drone flyovers, added that the drone had no affect on the security or functioning of the facility.

The latest flyover brings to 19 the number of unidentified drones spotted over nuclear plants since October 19 throughout France -- heavily dependent on nuclear energy with 58 reactors at 19 plants.

Police are clueless as to who is piloting these unmanned aircraft at a time of heightened vigilance in the face of Islamist extremism.

French law bans small, civilian drones from areas such as nuclear facilities, which are protected by a no-fly zone that spans a 2.5-kilometre (1.6-mile) radius and a height of 1,000 metres.

State-run power company EDF was the first to ring the alarm bells when it announced it had filed a complaint with police after detecting the small unmanned aerial vehicles zipping over seven nuclear plants last month.

Since then, more drones fitted with propellers have been spotted above nuclear facilities, and while experts agree these do not pose a threat to the rock-solid plants, they believe the mystery flyovers are being carried out to prove a point about nuclear security.

The environmental activist group Greenpeace, which has denied from the beginning being involved with the drones, has expressed concern about "the inability of state services to stop the flyovers".