The sightings came after several airspace violations by Russian drones in European skies and mysterious drones flying over Denmark late last month before two gatherings of European leaders in Copenhagen.

"Several drones were spotted by local police, both on the Belgian side and on the German side," the minister told broadcaster RTBF, adding that an investigation had been launched.

Francken said he did not know how many drones had flown or where they came from.

The drones flew over Elsenborn military base overnight Thursday to Friday, a spokeswoman for the defence minister told AFP.

"We are checking, we are looking into what happened, but at this stage, we don't have more details," she added.