A spate of mystery drones flying over French nuclear plants has led the country to launch a programme aimed at developing ways of detecting and intercepting them, officials said Thursday.

Around 20 unidentified drones have been spotted over nuclear plants since October 19 throughout France.

Police are clueless as to who is piloting the unmanned aircraft at a time of heightened vigilance in the face of Islamist extremism. France is heavily dependent on nuclear power.

"While the currently listed overflights do not present a threat to the operation and security of nuclear installations, they nevertheless constitute a warning of the potential risks from inappropriate or malicious use," a statement from France's General Secretariat of Defence and National Security said.

The secretariat has put France's National Research Agency in charge of developing the 1-million-euro programme ($1.2 million) aimed at finding ways to detect and intercept such drones.

France plans to share its findings with other European countries.

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.

Experts say they do not pose a threat to the rock-solid plants and believe the mystery flyovers are being carried out to prove a point about nuclear security.