Denmark's Aalborg airport was closed for three hours due to drone incursions late on Wednesday Photo: AP PHOTO

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.

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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.

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