COPENHAGEN, Denmark (AP) — Drones flew over four Danish airports overnight Wednesday into Thursday, the latest instance of unexplained drone activity that has raised concerns about security in northern Europe amid growing Russian aggression.

Defense Minister Troels Lund Poulsen called the incidents a “hybrid attack” because the flights all occurred within roughly the same timeframe. It was not immediately clear who was behind the incidents, but Poulsen said that it appeared a “professional actor” was behind the “systematic” flights. He did not provide more details during a news conference Thursday morning.

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