French security chiefs are investigating a spate of mysterious and illegal overflights of French nuclear power stations by tiny, unmanned drones.

A government official told The Associated Press that authorities have counted about 15 drone flights over a half-dozen nuclear sites since October 1. Authorities insist France's nuclear facilities are designed to handle seismic and security risks, including those possibly posed by drones.

Environment Minister Segolene Royal told various media Sunday that investigators have no leads about who is behind the flights.

Officials are keeping quiet about their response. Interior Minister Bernard Cazeneuve told RTL radio Monday that "the best way to be effective in this area is not to say what we're doing."

France gets more than two-thirds of electricity from nuclear power, the highest proportion in the world.