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.

A night of drone incidents at airports and military bases all over Jutland, western Denmark, has not caused any harm or damage - and yet it has exposed the country's defences as vulnerable to attack. In an era of hybrid warfare, there is a sense of embarrassment in Denmark - a founding member of the North Atlantic Treaty Organization (Nato) alliance - that its critical infrastructure has become so vulnerable. Aalborg and Billund airports had to close, while drones were spotted at Esbjerg, Sonderborg and Skrydstrup. Aalborg also serves as a military base and Skrydstrup is home to some of the air force's F-35 and F-16 war planes. Drones were also seen over the Jutland Dragoon regiment at Holstebro .