The mystery over a spate of unidentified drones spotted flying over French nuclear facilities deepened Friday after two men detained near a plant were released when police found they were harmless model enthusiasts.

French authorities have been left scratching their heads after detecting at least 18 flyovers throughout the country over the past month, and police are clueless as to who is piloting these unmanned aircraft at a time of heightened vigilance in the face of Islamic extremism.

In the latest incidents this week, a drone flew over the Saint-Alban atomic plant in the southeast on Wednesday night, and the next evening, an unmanned aircraft zipped over the Marcoule nuclear site just under 200 kilometres (124 miles) away.

On Wednesday, police detained two men aged 24 and 31 and a woman after they were found with two drones near the Belleville-sur-Loire nuclear plant in central France.

But prosecutor Vincent Bonnefoy later told reporters that their two drone flights near the power station "had no connection with the other nuclear sites overflown since October".

According to Bonnefoy, the trio said they wanted to use the drones to take pictures of a model boat they were floating on a lake near the plant.

He added the trio were "model enthusiasts" with no police record.

The 21-year-old woman was released Thursday, and the two men walked free on Friday although they were charged with flying an aircraft in a prohibited zone and have been banned from going to Belleville-sur-Loire, prosecutors said.

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 last week when it announced it had filed a complaint with police after detecting the small unmanned aerial vehicles zipping over seven atomic plants in October.

Since then, more drones fitted with propellers have been spotted above nuclear facilities, and while experts agree these do not pose a threat to the rock-solid plants, they believe the mystery fly-overs are being carried out to prove a point about nuclear security.