The Swedish Security Service (Sapo) said in a statement that it had taken over the probe from the police regarding "the events where drones have been sighted at Forsmark, Ringhals and Oskarshamn's nuclear power plants."

Forsmark is located some 140 kilometres (87 miles) north of Stockholm, while the other two are in the south of Sweden.

Swedish police said Saturday that it had received reports of drones flying over several nuclear power plants, adding that did not have a suspect at the time.

The nature of the flights meant that Sapo decided that they need to be investigated in closer detail and under the suspicion of "grave unauthorised dealing with secret information".

It did not share any details on potential suspects.

A drone was also spotted circling Sweden's parliament and government buildings and the royal palace in central Stockholm, according to newspaper Aftonbladet.

Citing an unnamed source close to the matter, the newspaper said the drone circling Stockholm was not a small commercial drone, but rather a winged military drone.

"This isn't about some small propeller drone, but a large winged drone with at least a two-metre wingspan," the source said.

In 2018, Greenpeace activists flew a drone fitted out as a flying Superman into a nuclear energy plant in southeast France, aiming to show how the country's reactors are vulnerable to terror attacks.

In 2014 and 2015, drone flights were reported over several French nuclear plants, but Greenpeace denied any involvement in those incidents.