Denmark's Aalborg airport was closed due to drones in its airspace, police said early on Thursday, two days after the country's main Copenhagen airport was shut over drone sightings that raised European security concerns.

The Danish national police stated that the drones followed a similar pattern to the ones that had halted flights at Copenhagen Airport for four hours a few days earlier. Local police later said the drones had left the Aalborg area after about three hours.

Denmark said on Tuesday the incident at Copenhagen airport was the most serious attack yet on its critical infrastructure.

The airport closure forced three incoming flights to be diverted to other airports.