FILE PHOTO: General view of nuclear power plant in Forsmark
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.
Comment: This brings to mind a number of other events that may be linked, and occurred in just the last few months:
- First-ever known drone attack against US electricity grid, DHS reports
- Undersea cable in Norway damaged, antenna station provides unique support to polar-orbiting satellites
- Finland & Sweden in NATO would trigger response - Russia
- American diplomats in Geneva, Paris fall victim to 'Havana Syndrome'
- Blast & fire at Gothenburg apartment block may have been explosive device, gas leak ruled out, police say
In addition, these unexplained drone sightings near sensitive sites has been ongoing for years now: