Drones have been seen near military facilities including Denmark's largest, following a series of incidents that caused air disruption earlier this week.

The devices were observed above Karup airbase, among others, forcing it to briefly close its airspace to commercial traffic. Possible sightings were also reported in Germany, Norway, and Lithuania.

It is the latest in a string of suspicious drone activity in Denmark, raising concerns about the nation's vulnerability to aerial attack and sparking fears of potential Russian involvement.

Danish authorities said Thursday's incursions appeared to be a hybrid attack, but cautioned that they had no evidence to suggest Moscow was behind it.

Friday's incident took place around 20:15 local time (18:15 GMT) and lasted several hours, according to duty officer Simon Skelsjaer who spoke with AFP. He noted police could not comment on where the drones had come from as they had not shot them down, and added that police were co-operating with the Danish military in their investigation.

Though civil airspace above the base was briefly closed, it had no impact because no commercial flights were scheduled in the area at the time.

The Danish Defence Ministry confirmed that drones were sighted near multiple military installations overnight, but did not specify which ones. The other military installations have not been named by Danish state media.

Some 3,500 people work at Karup airbase, which is home to all of the Danish Armed Forces' helicopters, airspace surveillance, and parts of the Danish Defence Command.

The incursions came just days after drones forced the shutdown of airports housing military facilities, including Aalborg and Billund airports.

German authorities are investigating a possible link to drone sightings in Schleswig-Holstein, which borders Denmark, while police in Norway are probing reports at Orland airbase. Drone activity also delayed flights at Vilnius airport in Lithuania.

Amidst the ongoing incidents, speculation lingers over the possibility of Russian indirect aggression targeting NATO allies, although such connections remain unproven.