Denmark faces “systemic” drone incidents brought by a “professional” actor seeking to disrupt airports and military installations, defence minister Troels Lund Poulsen said.

Authorities have no evidence to confirm who is behind the incidents as they continue investigating the drone sightings.

But Poulsen noted that “there are countries or actors who may have an interest in undermining our support for Ukraine” as he insisted “it is crucial we do not allow ourselves to be intimidated by Russia.”

The drones appear to be potentially getting released in close vicinity to targets, posing additional questions about how they are operated and safety of critical infrastructure.

The incidents appear to seek to “test boundaries, and create fear,” justice minister Peter Hummelgaard said, as he warned that “the threat of hybrid attacks is something here to stay.”

Denmark is considering triggering the Article 4 of Nato treaty to convene a meeting to discuss the threat, after Poland and Estonia chose to do so earlier this month, but no final decision has been made yet.

Ministers warned the nature of the incidents make it “completely unsustainable” for Danish infrastructure to be regularly put out of use, as they pledged to step up the country’s defences against drones.

Officials were particularly pressed on their decision not to shoot the drones down, particularly over the Skrydstrup airbase which hosts F-35 fighter jets, arguing it was based on operational assessment at the time, but conceded they needed to get better at responding to drone incidents.

On Friday, Denmark will join an EU meeting on the so-called “drone wall” to discuss regional cooperation and lessons that could be learned from Ukraine.

Resolving the issue is increasingly urgent as EU leaders are expected in Copenhagen next week for an informal European Council meeting.