German Defense Minister Boris Pistorius said on Thursday that Europe is "not at war" and yet "no longer at complete peace either", in response to the increasing appearance of drones over several European countries.

"We are under attack, hybrid attacks in the form of disinformation campaigns and, indeed, drone incursions", he added.

Pistorius also said that even though the Danish Prime Minister Mette Frederiksen could not logically say, the obvious suspicion was that the appearance of drones over several airports in Denmark was part of Russian President Vladimir Putin's strategy.

Drone flights come as no surprise to him, Pistorius said on the sidelines of the conference of East German state premiers at Ettersburg Castle near Weimar.

Drones flew over four Danish airports overnight, the latest instance of unexplained drone activity that has raised concerns about security in northern Europe amid growing Russian aggression.

Authorities say it was not immediately clear who was behind the incidents but there is no danger to the public.

A similar incident at Copenhagen Airport grounded flights for hours Monday night, prompting concerns that Russia could be behind it.