Drone incursions that briefly shut two of Denmark's airports and affected military installations were hybrid attacks intended to spread fear, authorities say, but they do not know who was behind them.

The incidents are just the latest in a series of drone incursions - including two days ago in Denmark - in the past few weeks that have exposed the vulnerability of European airspace and the challenges governments face in countering them.

Poland shot down suspected Russian drones in its airspace on September 10.

Danish authorities said on Thursday they decided not to take down any of the drones in its airspace for safety reasons, despite the disruption caused to air traffic.

Billund airport, Denmark's second biggest, was closed for an hour, and Aalborg airport, used for commercial and mil

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