A drone has flown over a nuclear plant in central France for the second time in two days, a source close to the case said Monday, the latest in a mysterious string of such incidents.

The unmanned flying machine went over the plant in Dampierre-en-Burly on Sunday evening, said the source, who wished to remain anonymous. The power station had already been overflown on Friday evening.

Authorities are scratching their heads over the number of unidentified drones spotted over nuclear plants across France over the past month, and a probe has been launched to try and find out exactly who is piloting the remote-controlled machines.

They have counted at least 15 fly-overs, sparking questions over the security of nuclear plants in France, which relies heavily on atomic 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".

But it is against French law to fly within a five-kilometre (three-mile) radius of a nuclear plant. Anyone breaking the law is liable to one year in prison and a fine of 75,000 euros ($94,000).

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 ecologist Green Party before the 2012 parliamentary and presidential elections, President Francois Hollande's Socialists promised to cut reliance on nuclear energy from more than 75 percent to 50 percent by shutting 24 reactors by 2025.