STOCKHOLM, Sweden: Sweden's security service is investigating drone sightings reported over key Swedish sites, including nuclear plants.

Sightings were reported at three nuclear sites, as well as over airports and the royal palace, but authorities have not speculated on who is responsible.

Local media reported police and the coast guard are searching the sea and islands around Stockholm.

The latest sightings involved a drone above the Forsmark nuclear plant, but a security agency said it was also investigating earlier drone flights near the Ringhals and Oskarshamn power plants.

Police appealed to the public to come forward with information.

A security agent said the drones were suspected of "grave unauthorised dealings with secret information."

Swedish TV reported a police helicopter was following a drone flying above it at a height of up to 1,000m near the capital, Stockholm.

According to the Aftonbladet news site, a drone was seen over the weekend circling the parliament and government buildings, as well as the royal palace in Stockholm. Drones were earlier spotted near Kiruna and Lulea airports.

The drone sightings come at a time of increased military alert in Sweden because of tensions between Russia and the West over the Russian military build-up along the border with Ukraine.

Sweden recently deployed troops to the Baltic island of Gotland, described by officials as a signal that Sweden's military forces are ready to defend their territory.

Sweden is not a member of NATO, but said it was responding to three Russian landing craft moving into the Baltic Sea through Denmark's Great Belt Strait.