It was not immediately clear who was flying the drones. The airports were closed for several hours, disrupting dozens of flights.

Mysterious drone sightings in western European countries like Denmark, Norway and Poland are raising concerns across the continent, with the NATO alliance vigilant about the threat of Russian aggression.

Copenhagen Airport , the busiest in the Nordic region, said it reopened early on Tuesday after drone sightings halted all take-offs and landings for nearly four hours, with Norway's Oslo Airport also re-opened after it had shut its airspace over a drone.