Police in Sweden deployed patrols and helicopters to the Forsmark nuclear plant to hunt for a large drone seen flying over the site late on Friday, but were unable to catch the unmanned vehicle, they said on Saturday.
The incident came a day after Sweden's military started patrolling the main town on the Baltic Sea island of Gotland amid increased tensions between NATO and Russia and a recent deployment of Russian landing craft in the Baltic.
Forsmark, which is Sweden's biggest electricity producer, lies on the Baltic coast about 150 km (93 miles) north of the capital, Stockholm.
Police saw the drone, first spotted by a security guard, moving around the plant before disappearing towards the island of Graso.
"Police continue to try to locate the drone, even with their own (drones), but without success," the police force said in an incident report on its website, adding that there were no signs the drone had dropped anything in the area or landed.
It also cited unconfirmed reports of possible drone sightings at the Ringhals nuclear plant on the country's west coast.