"We do not have information allowing us to name those responsible for the events of recent days," the head of the service, Thomas Ahrenkiel, told a press conference -- though intelligence chief Finn Borch said: "The risk of Russian sabotage in Denmark is high."

"We can say that this resembles a pattern of hybrid warfare that we have seen elsewhere in Europe," Borch added.

Drones were spotted overnight Wednesday to Thursday above the airports of Aalborg in the north, Esbjerg in the west, Sonderborg in the south and the Skrydstrup military air base in the same region, according to police.

More drones of unknown origin had already flown over Copenhagen airport on Monday, halting traffic for several hours.