Brussels: An unexplained drone has been spotted flying over a Belgium nuclear facility, a day after one of the plant's reactors came back on line after a four-month closure caused by sabotage.

The mystery appearance by an unmanned aircraft on Saturday, on which Belgian authorities refused to provide much detail, resembles a spate of similar drone sightings over nuclear plants in neighbouring France this autumn.

Around 20 unidentified drones have been spotted over nuclear plants since October throughout France.

"We can confirm that the East Flanders prosecutor's office has opened an investigation into a drone flight over the Doel nuclear plant," a spokesman for the investigation told Belga news agency.

"We will not provide further information for the time being," the spokesman added, hours after the plant's operator, GDF-Suez unit Electrabel, first disclosed the incident, which took place early on Saturday.