Major Danish airports were briefly closed early Thursday after drones were spotted, as ongoing airspace breaches rattle Europe.

Danish authorities responded overnight to drone sightings at Aalborg airport in the country’s north, along with airports at Billund, Esbjerg, Sønderborg and Skrydstrup further south, closing airspace and canceling flights.

Aalborg’s airport reopened early Thursday morning and air traffic has resumed. Danish chief inspector Jesper Bøjgaard Madsen said he could not “comment on the purpose of the drones flying in the area,” nor “who the actor behind it is.”

“If we get the opportunity, we will take down the drones,” he added.

In another statement, Danish police in the country’s south said “several drones with lights were observed” at Esbjerg, Sønderborg and Skrydstrup, but it was not “possible to take down the drones or find the drone operators,” and those three airports were not closed. Skrydstrup is an air base and hosts Denmark’s fleets of F-16 and F-35 jets.

Billund Airport, which is one of Denmark’s busiest cargo centers, was closed for about an hour after a drone sighting and quickly reopened, authorities said.

Both Copenhagen and Oslo airports were shut down late Monday after drones were spotted in their airspace, forcing flights to be diverted or canceled and stranding thousands of passengers.

Danish Prime Minister Mette Frederiksen said Tuesday she could not “rule out in any way that it is Russia” behind those drone incidents, calling it “the most serious attack on Danish critical infrastructure to date.”

In recent weeks, Estonia and Poland convened NATO members for urgent talks after Russia was accused of violating their airspace in separate incidents. Moscow has denied responsibility for the Estonian incursion and said the Polish incident was an accident.

At the United Nations this week, U.S. President Donald Trump suggested NATO countries should shoot down Russian aircraft breaching their airspace.