(CNN) — Two of Scandinavia’s busiest airports – Oslo and Copenhagen – were forced to close for several hours due to drone sightings overnight into Tuesday, disrupting the travel plans of tens of thousands of people.

The airspace over Denmark’s Copenhagen Airport was closed for almost four hours on Monday night after two to three large “unidentified” drones were spotted in the area. No flights wereallowed to take off or land.

“We have concluded that this was what we would call a capable operator,” Danish police Chief Superintendent Jens Jespersen told reporters on Tuesday, Reuters reported.

The drones came from several different directions, turning their lights on and off, before eventually disappearing after several hours, Jespersen said.

“It’s an actor who has the capabilities, the will and the tools to show off in this way,” he said, adding that no suspects have yet been identified.

Copenhagen airport, which serves nearly 30 million people each year, reopened around 12:20 a.m. local time (6:20 p.m. ET) but some delays and cancellations were still expected, an airport spokesperson said early Tuesday.

Earlier, arriving flights wereredirected to other airports in Denmark, while some were sent to Gothenburg and Malmö in Sweden, according to the airport.

In Norway, the airspace over Oslo Airport was closed for around three hours? Tuesday morning due to a separate drone sighting. “This means that incoming flights are now being redirected to the nearest airport,” Communication Manager Monica Iren Fasting told CNN.

Police were investigating the incident, she added.

Oslo Airport reopened around 3:22 a.m. local time (8:22 p.m. ET), Reuters reported, citing a spokesperson for Norwegian airport operator Avinor.

In an earlier incident in Oslo on Monday, Police arrested two foreign nationals for flying drones over a restricted area. Norwegian media reported the drones were flown over the Akershus Fortress, a medieval castle that is sometimes used for government events. There was no immediate indication it was connected to the events at the airport or in Copenhagen.

Europe has been on alert since Russian drones violated both Polish and Romanian airspace earlier this month – prompting NATO allies to pledge to beef up defenses on the bloc’s eastern flank. There is currently no indication that Monday’s drone activities in Denmark and Norway are connected to those incidents involving Russia.

This story has been updated.

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