Denmark says drones flown over its airports appear to be the work of a "professional actor" but there was no evidence of Russian involvement, after such activity disrupted its airspace for the second time in a week.
Aalborg airport was forced to shut down for several hours on Wednesday evening after green lights were spotted overhead, while Billund airport also briefly closed. Three smaller airports also reported drone activity.
On Monday, Copenhagen airport temporarily closed due to a drone incursion.
Defence Minister Troels Lund Poulsen told a news conference that the "hybrid attack" was part of a "systematic operation", but said the devices had been launched locally.
A hybrid attack uses a mix of military and non-military tactics and is designed to interfere with a nation's infrastructure or institutions.
Drones pose a risk of colliding with planes, particularly during take-off or landing, but halting air traffic in and out of an airport to avoid this can cause delays and flights being redirected elsewhere.
At least three flights were diverted away from Aalborg during the incident.
All the airports affected by the overnight incident sit on Jutland, the part of Denmark that is on the European mainland.
Drones were first sighted over the northerly Aalborg airport around 21:44 local time (19:44 GMT), state media reports. A few minutes later, police received reports of drone activity near small, southerly airports in Esbjerg, Sønderborg and Skrydstrup.
Possible drone activity over Billund prompted the closure of its airport for about an hour early on Thursday morning.
The drones were assessed to have cleared shortly before 03:00 and the airspace was reopened. Danish police are now seeking to find out who launched them.
Poulsen said there was no evidence to suggest Russia was behind the incursion. The Russian embassy in Copenhagen has denied "absurd speculations" of its involvement.
It described the incidents as "staged provocations", saying they would be used "as a pretext for further escalating tension" over the war in Ukraine.
Russia has been accused of conducting hybrid attacks in the past, and Europe has been on high alert after several Nato member states reported Russian incursions in their airspaces.
Estonia and Poland requested a consultation with other Nato members last week, after around 20 Russian drones crossed into Poland and Russian MiG31 jets entered Estonian airspace in a separate incident. Romania, another Nato member, also said a Russian drone had breached its airspace.
Russia denied violating Estonia's airspace, while it insisted the Polish incursion was not deliberate. It did not comment on the incident in Romania.
Suspicious drones have also been sighted over Germany and Sweden. All are suspected - but not proven - to be part of Russia's indirect aggression towards Nato states supporting Ukraine.
Danish Prime Minister Mette Frederiksen said Russian involvement in the Copenhagen drone incursion could not be ruled out, describing it as "the most severe attack on Danish infrastructure so far". The incident also saw Oslo airport in Norway temporarily close.
Kremlin spokesman Dmitry Peskov called the allegations "unfounded".
Officials said the Danish Armed Forces had been affected by Wednesday's incident, as Aalborg airport is also used as a military base. Skrydstrup is also home to an airbase.
The drones were not shot down, despite Danish authorities saying they had the ability to do so. Officials said this was due to concerns for the safety of the surrounding population.
"If we get the opportunity, we will take down the drones," chief inspector Jesper Bøjgaard Madsen said.
Police said they did not believe the drones posed any danger to people at the airport or nearby residents, but asked the public to keep their distance from the area.
EPAPeter Hummelgaard, Denmark's justice minister, said the hybrid threats were "here to stay".
Poulsen noted that an EU meeting had been called for Friday, at which enhanced drone preparedness across Europe would be discussed.
Earlier this month, European Commission President Ursula von der Leyen called for a "drone wall" to tackle incursions launched by Russia.
What that means is not entirely clear - but detecting the drones early and then destroying them are seen as key elements of such a strategy.
US President Donald Trump has gone as far as to say that Nato nations should shoot down Russian planes in their airspace, while Nato has warned it would use "all necessary military and non-military tools" to defend itself following the recent military incursions.
"We are a defensive alliance, yes, but we are not naive, so we see what is happening," Nato's secretary general Mark Rutte said.
