Unidentified drones were observed during the night over Denmark’s biggest military base, police said on Saturday, the latest in a slew of sightings that Danish officials have called a “hybrid attack”.

The mystery drone observations have prompted the closure of several airports across the Scandinavian country since Monday, when the first drones appeared.

“I can confirm that we had an incident around 8:15pm (11:15pm PKT) that lasted for some hours. One to two drones were observed outside and over the airbase,” duty officer Simon Skelsjaer told AFP, referring to the Karup military base.

He said police could not comment on where the drones came from, adding: “We didn’t take them down.”

Police were cooperating with the military in their investigation, he said.

The Karup base shares its runways with the Midtjylland civilian airport, which was briefly closed though no flights were affected as no commercial flights were scheduled at that hour, Skelsjaer said.

Danish Prime Minister Mette Frederiksen said on Thursday that “over recent days, Denmark has been the victim of hybrid attacks,” referring to unconventional warfare.