By Sophie Tanno, CNN

(CNN) — Multiple airports in Denmark have been disrupted by drones this week in what its government says is a “hybrid attack” by unknown professional actors, as the country considers whether to invoke NATO’s Article 4 for the first time in its history.

Speculation is rife that Moscow could be behind the incursions, as Danish authorities say they are linked with recent alleged violations of NATO airspace in Poland and Romania by Russian drones.

Russia’s Embassy in Denmark has denied any Russian involvement, saying Thursday that it rejects the “absurd speculation.”

Europe is already on high alert over the series of airspace breaches by Kremlin drones and aircraft. Poland requested Article 4 consultations with NATO after Russian drones penetrated its airspace and that of Romania in separate incidents earlier this month, while Estonia also did so after Russian MiG31 fighter jets entered its airspace without permission on Friday. Article 4 requires NATO states to hold talks when “the territorial integrity, political independence or security” of a member is threatened.

What took place during this week’s drone sightings in Denmark – and what are the potential risks?

What’s happened?

On Monday, sightings of two or three large drones near Copenhagen airport halted all takeoffs and landings there for nearly four hours.

Two days later, on Wednesday night, drones forced the closure of a second Danish airport, when Aalborg airport in the country’s north was shut for three hours after “more than one drone” was seen flying nearby. Drones were also observed overnight into Thursday near Danish airports in Esbjerg, Sonderborg, and Skrydstrup, according to Danish news outlet TV2. Billund Airport was also briefly closed due to reports of drone activity.

In neighboring Norway, the airspace over Oslo Airport was closed for around three hours Tuesday morning due to a separate drone sighting.

The incidents follow a major cyberattack at a provider of check-in and boarding systems over the weekend, which disrupted operations at several of Europe’s busiest airports, including London’s Heathrow.

Who is behind the drones?

We don’t know yet – and Danish authorities say it’s too soon to tell. However, the country’s top officials have suggested Russia could be to blame.

At Thursday’s news conference, Denmark’s Minister of Defense Troels Lund Poulsen acknowledged the incursions must have been carried out by a “professional actor.” He said that it was part of a “systematic operation,” but the drones had been launched locally. Poulsen added that there is currently no evidence that Russia is behind the incidents.

Earlier, Prime Minister Mette Frederiksen described Monday’s incident at Copenhagen airport as the most serious “attack” yet on Denmark’s critical infrastructure, and linked it to the drone breaches by Russia in Poland and Romania.

If Russia is behind this, a key question remains over who exactly is launching the drones; Russian security services themselves, or criminal elements acting on behalf of the Russian state.

The concept of Russia recruiting criminal actors on European soil is nothing new. In his annual speech on security threats facing the UK in October 2024, Ken McCallum, director-general of British intelligence service MI5, warned that state actors – particularly Russia and Iran – are increasingly recruiting criminal elements including petty criminals to carry out hostile acts on British and European streets.

Little is known yet about what kind of drones were used in Denmark’s sightings. However, the use of high-tech drones is becoming more widespread as the technology develops, making it difficult to draw conclusions as to those responsible.

“Drone technology has become very sophisticated,” said Edward Arnold, a senior research fellow for European security at UK defense and security think tank RUSI. “A lot of people are trained and use drones almost for recreational purposes, so this isn’t now the preserve of elite, state-based actors,” Arnold said.

What’s the motive?

Denmark says the main intention with such incidents is to sow fear.

“The goal with this kind of hybrid attack is to create fear and divisions, and make us scared,” Justice Minister Peter Hummelgaard told Thursday’s conference.

Hybrid attacks, according to NATO, are used to “blur the lines between war and peace.” Methods include propaganda, deception, sabotage and other non-military tactics, the alliance says.

Arnold says Russia would have reason to single out Denmark specifically.

“Their (the Danish) rhetoric is very anti-Russian at the moment due to the war in Ukraine. They are a steadfast supporter of Ukraine and over the last couple of years they have done more than most European nations. If you look at it by GDP, for a small nation they are one of the leaders in Europe for supporting Ukraine,” he told CNN.

Therefore, he believes there could be a Russian “signaling” element to it, with Moscow trying to warn Copenhagen off its support for Ukraine.

If Russia is behind the attacks, the goal could simply be to cause disruption.

“The Russians do this all the time. They’re happy just disrupting and being a nuisance, as a baseline,” Arnold said.

Another, more serious, long-term concern is that, while the drone incursions this week have been relatively minor and aimed at disruption, they could be a practice run, or a “perfection of the technique,” for causing more serious damage in future, potentially with armed drones, Arnold said.

Going forward, it’s not implausible that similar drone sightings and disruptions could be seen in other countries.

“What do the Russians, and Iranians for that matter, do when they are successful at certain operations? They replicate them,” Arnold said.

They are also more likely to occur at times which cause maximum disruption, such as around Christmas and summer holidays, he believes. “Criminal elements, drone pilots being a nuisance, it is a very difficult area to protect against,”

How can they be stopped?

As well as closing affected airports, there are other measures that can be taken to protect against drone disruptions.

Shooting down drones is possible – although this was ruled out in Denmark this week after the armed forces assessed it would be too risky for civilian populations.

Shooting down drones runs the risk of missing the target, Arnold explained. “If that shot misses the drone, that bullet is going to go somewhere else, and it could pose a risk to the public.”

In comparison, if a drone was armed and had the ability to cause widespread damage, armed forces would be much more likely to take action to shoot the device down.

Another option is drone jamming – which uses electronic signals to disrupt the device’s contact with its operator and can render it uncontrollable or force it to land.

Other methods include sending counter-drones with nets to capture the devices, and even training eagles and other birds of prey to take them down. In 2016, it emerged that Dutch police were deploying eagles after training them to take down unauthorized drones.