French authorities said on Friday they had detected drones over two nuclear power plants, the latest in a baffling series of incidents across the country.

A spokesman for security forces said: "Drone-type machines overflew two nuclear plants during the night. They were detected by police in charge of protecting the plants and staff."

"These machines were not neutralised because they did not represent a direct threat" to the nuclear facilities, the spokesman added.

National energy company EDF said on Wednesday it had identified seven drones flying over its plants this month and had filed a complaint with the police.

A spokeswoman for EDF confirmed the latest incident, which she said happened around 9:00 pm local time over plants at Penly in northern France and Golfech in the southwest.

An enquiry has been launched, with a source close to the probe saying they were trying to find the pilot of the remote-controlled drones.

The drone flights have sparked questions over security of nuclear plants in France, which relies heavily on nuclear energy for electricity.

The interior ministry has stressed that a drone does not pose any concern for the plants which are "designed to withstand a strong earthquake or an airliner crashing into it."

It is against French law to fly within a five kilometre (three mile) radius of a nuclear plant. Those breaking this law are liable to one year in prison and a fine of 75,000 euros ($93,000).

Environmental lobby group Greenpeace, whose activists have in the past staged protests at nuclear plants in France, has denied any involvement in the mysterious pilotless flight activity.

France, the world's most nuclear-dependent country, operates 58 reactors and has been a leading international cheerleader for atomic energy.

But in a deal with the Greens before the 2012 parliamentary and presidential elections, President Francois Hollande's Socialist party promised to cut reliance on nuclear energy from more than 75 percent to 50 percent by shutting 24 reactors by 2025.

burs-ric/yad