Airports in Denmark and Norway have had to close after reports of drones in their airspace in recent days.
The incidents are part of what some European officials see as a pattern of Russian disruption that has exposed the vulnerability of European airspace at a time of high tensions between Moscow and NATO.
But the Kremlin has strongly denied any involvement.
Here is all you need to know.
What has happened?
In Denmark, the civilian-military Aalborg airport was shut overnight on Wednesday into Thursday, and planes grounded. Danish police said the drones followed a similar pattern to the ones that caused a shutdown at Copenhagen airport a few days earlier.
Police said drones had also been observed near airports in the Danish towns of Esbjerg, Sonderborg and Skrydstrup – where Denmark’s fleet of F-16 and F-35 fighter jets are based.
In Norway, authorities were forced to halt flights at Oslo airport for three hours on Monday after a drone was seen.
A link between the Norwegian and Danish incidents has yet to be established.
It comes after reports of Russian drones breaching the airspace of Poland and Romania, prompting Article 4 conversations within NATO.
Why is it dangerous?
Danish justice minister, Peter Hummelgaard, said on Thursday the goal of the drones was to “sow fear and division”.
When drones fly over airports, they pose a risk of colliding with planes, particularly during take-off or landing.
In 2018, researchers at the University of Dayton Research Institute released video footage of what can happen when a small drone strikes the wing of an aircraft.
The team mimicked a collision between an unmanned aircraft and a small Mooney M20 aircraft at 238mph.
The footage showed the drone ripping a hole through the wing.
Do airports have to completely close?
No, airports do not have to implement a complete shutdown when a drone is detected, but it is typical to halt air traffic in and out of the space to avoid collisions.
This has knock-on effects for travelling passengers, causing delays and flights being redirected elsewhere.
It is the owner or operator of an individual airport that decides whether to close the facility, and for how long planes are halted for.
Is Russia responsible?
There has been a range of drone incidents reported and it’s not clear yet how many – if any – are linked.
Danish national police commissioner Thorkild Fogde said many people around the country had reported drone sightings to the police since Monday.
“Of course, many of these reports do not cover activities that are of interest to the police or the military, but some of them do, and I think the one in Aalborg does,” he said.
“It is too early to say what the goal of the drones is and who is the actor behind,” a Danish police official from the north of the country said.
Of the Copenhagen incident, European Commission President Ursula von der Leyen said the drones were part of a “pattern of persistent contestation at our borders”.
Russia’s embassy in Copenhagen rejected suggestions that Moscow could be involved, calling the accusation “absurd” speculation.
Kremlin spokesperson Dmitry Peskov also denied any involvement.
Russia is regularly accused by European countries of engaging in ‘grey zone’ or hybrid warfare, something it’s leadership persistently denies.
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Has this happened before?
A string of drone sightings as well as digital outages has repeatedly disrupted airports since 2018.
In December 2018, Gatwick Airport in West Sussex – the UK’s second-busiest airport – was forced to shut down a runway for almost 30 hours after more than 100 drone sightings over three days.
More than 100,000 Christmas travellers were affected by the airport’s closure, in what was the first time a major airport in the UK had been shut down due to drones. Two people were later held but released and subsequently exonerated. Police maintain it was a malicious incident.
Almost five years after that, in 2023, Gatwick was again forced to shut down a runway for almost an hour as police investigated a drone sighting.
Last year, the operator of Frankfurt airport in Germany – one of Europe’s busiest – said that air traffic had to be stopped completely on two days in 2023 over drone incursions.
While, in America, drone sightings last year forced officials to close down Wright-Patterson Air Force Base near Dayton, Ohio, for around four hours.
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What can airports do to stop drones?
After the recent sightings, the Danish defence minister said the country would get new capabilities to neutralise drones and put forward laws to allow airports to shoot them down.
Similar legislation was implemented in the UK In the wake of the 2018 incident at Gatwick.
The UK government introduced extended no-fly zones around airports to three miles (around 5km) and millions of pounds was invested in anti-drone technology which was installed at both Gatwick and London Heathrow.
Known as the Israeli-designed Drone Dome system, the technology can jam communications between a drone and its operator, while it also uses a laser beam in order to locate and destroy hostile drones.
It is said to be able to intercept drones at a range of several kilometres.
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Russian airports and airfields, which are frequent targets for Ukrainian kamikaze drones, regularly utilise GPS jamming to try and disorientate UAVs and stop them from finding their targets.
However, this is an imprecise technology and is not targeted, meaning that other aircraft in the surrounding area would also be affected and thus likely impractical for European countries that are not involved in a direct conflict.
