Drone Flights 'Most Serious Attack' on Danish Infrastructure, PM Says

Airports in Copenhagen and Oslo reopened early Tuesday, hours after unidentified drones in their airspace caused dozens of flights to be diverted or canceled, disrupting thousands of passengers.

"This is part of the development we have recently observed with other drone attacks, airspace violations, and cyberattacks targeting European airports," Frederiksen said in a statement.

Danish intelligence said the Scandinavian country was facing a "high threat of sabotage."

"Someone may not necessarily want to attack us, but rather stress us out and see how we react," Flemming Drejer, director of operations at Denmark's intelligence service PET, told a press conference Tuesday.

Police said that they had been unable to identify the drone operator.

"The number, size, flight patterns, time over the airport. All this together indicates that it is a capable actor. Which capable actor, I do not know," Copenhagen police inspector Jens Jespersen told reporters.

"It was an actor that had the capacity, the will and the tools to make their presence known," he said.

Airport officials said air traffic had resumed early Tuesday but 20,000 passengers were affected by 31 flight diversions and more than 100 cancelations.

Heavy delays and disruptions were expected throughout Tuesday as many planes were not at their planned airports.

Copenhagen police said earlier they were also cooperating with colleagues in Oslo after drone sightings in the Norwegian capital also caused the airport to close for several hours.