Authorities have been alerted after unidentified drones were spotted flying over seven Électricité de France (EDF) nuclear power plants in France during October 2014.
Despite staging protests at the same power plants in the past - and using a drone to record them - environmental group Greenpeace has denied any involvement with the suspicious activity and shares the energy company's concerns at the evidence of a "large-scale operation."
A Greenpeace spokesperson told IBTimes UK: "We had no involvement in those flights, and we always claim responsibilities for our actions."
Drones were spotted flying over seven EDF power plants spread across the country between 13 and 20 October, usually at night or in the early hours of the morning.
On 19 October drones were spotted at four different plants - Bugey in the east, Gravelines and Chooz in the north, and Nogent-sur-Seine in north-central France.
The sites are hundreds of miles apart, suggesting the surveillance is being carried out by more than one person. EDF says it had notified the police on every occasion, but so far neither the security forces or the French Nuclear Safety Authority have commented publicly.
'We are very worried'
Yannick Rousselet, head of Greenpeace's anti-nuclear campaign, said in a statement sent to the AFP news agency: "We are very worried about the occurrence and the repetition of these suspicious overflights."
The Bugey plant has previously been a target for Greenpeace, as in May 2012 an activist flew a paraglider over the power station and landed on the site to highlight alleged security weaknesses; a drone was used to film the stunt.
Under French law, aircraft are not allowed to fly below 1,000 metres within a five-kilometre radius of nuclear power plants.
Earlier this week, it was revealed that a drone was "deliberately" flown too close to a passenger plane coming in to land at Southend airport in Essex. The pilot reported that the drone had come within 25 metres of his 74-person capacity aircraft on 30 May, 2014. UK law states drones cannot be flown near airports, or within 150 metres of built-up areas.
The French drone sightings occurred in the same week that a football match between Serbia and Albania was abandoned after a drone carrying the 'Greater Albania' flag was flown into the stadium, sparking violence between fans.
Meanwhile in the US, the Federal Aviation Authority (FAA) this week clarified that a law making live sports stadiums and racetracks no-fly zones includes drones as well as conventional aircraft.