Unidentified drones seen over several of France's nuclear reactors in recent weeks prompted the French government to launch an investigation on Thursday.

"Measures are being taken to know what these drones are and neutralize them," French Interior Minister Bernard Cazeneuve told France Info Radio.

According to the state electricity company EDF, the unmanned aircraft were spotted over seven nuclear plants across the country between October 5th and October 20th, without any impact on the plants' safety or functioning.

It is not known who was behind the mysterious flights. Aircraft are not permitted to fly within a 5-kilometer (3-mile) radius and an altitude of 1,000 meters (3,280 feet) over nuclear plants.

In France, suspicion immediately fell on the anti-nuclear lobby. The environmental campaign group Greenpeace denied any involvement in the incident and called the drone sighting "very worrisome." Its activists have carried out several break-ins at nuclear plants in recent years in a bid to raise awareness about nuclear safety.

France's nuclear dependency

The incident has reignited concerns about the safety of nuclear plants in France, a nuclear-reliant country with 58 reactors on 19 sites operated by EDF. The reactors provide three-quarters of the country's power.

Greenpeace claimed that "at least ten" nuclear plants nationwide had been targeted by the drones, including the country's oldest nuclear plant in Fessenheim, at the German border. EDF denied any drone sightings over Fessenheim.

French President Francois Hollande plans to reduce the atomic share in the energy mix to 50 percent by 2025.

das/kms (dpa, Reuters)