Danish Prime Minister Mette Frederiksen on Tuesday said a drone incident that shut down Copenhagen Airport for several hours late Monday was "a serious attack on Danish critical infrastructure." The incident should be viewed within the broader context of hybrid threats across Europe, she added.

"We have seen drones over Poland, activity in Romania, violations of Estonian airspace, cyberattacks on European airports over the weekend, and now drones in Denmark and Norway," Frederiksen told reporters.

Russian involvement cannot be ruled out, she underlined, noting that Europe faces "a series of hybrid attacks and sabotage attempts."

Copenhagen's Kastrup Airport suspended operations for around four hours on Monday evening, affecting approximately 20,000 passengers. Delays and cancellations continued into Tuesday, with travelers missing study trips, honeymoons, business journeys and sports events.

The Danish National Police confirmed that the National Operational Staff, a special crisis response unit, has been activated at its lowest level, meaning heightened surveillance and inter-agency coordination. Danish police are treating the case as a drone attack.

In neighboring Norway, Oslo's Gardermoen Airport was also forced to close around midnight Monday after drone sightings. The airport reopened at around 3.30 a.m. on Tuesday morning. Norway's Police Security Service said it is in dialogue with Danish authorities to assess whether the two incidents are connected.

Several flights to Oslo were diverted to other Norwegian airports, leaving passengers stranded overnight. Airlines reported delays, cancellations and rerouted services throughout Tuesday. Avinor, the state-owned operator of Norwegian airports, said travelers could expect continued disruption.