Some 20  drones have been reported flying over the port of Koege, southwest of Copenhagen, Danish police said Saturday.

They said the origins of the drones remained unclear and that they notified Denmark's PET domestic intelligence service about the incident.

This incident comes amid heightened concerns in Nordic and Baltic states about potential Russian hybrid attacks following recent damage to several undersea cables.

Mysterious drones above Baltic port

Police were called to the marina in Koege on Friday night after an eyewitness reported seeing the drones.

Officers spotted four of the drones before the devices quickly vanished over the sea.

It remains unclear where they came from, why they were there, or who launched them.

At that time, no one had issued permits for drone flights in that area.

Drone sightings in the US, UK and Germany

Drones of unknown origin have often been seen flying over power stations and airports in Scandinavia in recent years. Authorities have not been able to identify who launched them.

In recent weeks, a large number of mysterious drones have been reported flying over the US state of New Jersey and across the eastern United States, sparking speculation and concern over where they came from and why.

In November, a number of unidentified drones were spotted overthree airbases in the United Kingdom used by the United States Air Force. German authorities also said unidentified drones had been spotted flying over sensitive military and industrial sites, including the US air base at Ramstein.

Concern over potential Russian sabotage

Friday's drone sightings in Koege come after multiple incidents involving undersea cables in the Baltic Sea. Koege is a port on the Baltic Sea.

Last week, the Estlink 2 undersea power cable linking Finland and Estonia was damaged.

Finnish authorities believe the incident may have been an act of sabotage and detained the Cook Islands-flagged oil tanker Eagle S, a ship that the EU believes could belong to the so-called Russian "shadow fleet."

The "shadow fleet" refers to ships that Russia uses to circumvent sanctions, such as oil sales.

In a separate incident in November, two fiber-optic cables in the Baltic Sea were severed. Swedish authorities boarded a Chinese ship as part of an investigation into potential sabotage.

As a deterrent, NATO has announced an increase in its military presence in the Baltic Sea.

lo/sms (AP, dpa)