Drones were spotted flying over four airports in Denmark overnight sparking fresh fears that Putin may be testing NATO defences.

Several unmanned aircraft were spotted at Denmark's airports in Aalborg, Esbjerg, Sonderborg and at the Skrydstrup air base before leaving on their own, police said.

Aalborg airport, located in northern Denmark and one of the country's biggest after Copenhagen, was shut down before reopening several hours later.

All arriving and departing aircrafts were halted on Wednesday night, with inbound flights diverted to other airports.

Police were unable to shoot down the drones at Aalborg airport, 'which flew over a very large area over a couple of hours', North Jutland chief police inspector Jesper Bojgaard Madsen said.

An investigation is underway with the Danish intelligence service and the armed forces to 'clarify the circumstances', while the drone operators are yet to be apprehended, police said.

The purpose of the drones remain unclear and it is unknown who was controlling them, they added.

The latest incidents in Danish skies follow similar events in Poland and Romania and the violation by Russian fighter jets of Estonia's airspace, which have raised tensions in light of Russia's ongoing invasion of Ukraine.

Donald Trump has urged NATO countries to shoot down intruding Russian planes, while foreign secretary Yvette Cooper described the incidents as 'provocative and reckless' and warned the UK was 'ready to act'.

A light moves in the sky over Aalborg, amid reports of drone sightings that led to the airport being closed for several hours on Thursday

South Jutland police said they had 'received several reports of drone activity at the airports in Esbjerg, Sonderborg and Skrydstrup', late Wednesday evening.

The Esbjerg and Sonderborg airports were not closed because no flights were scheduled there until Thursday morning.

Police there said the drones 'flew with lights and were observed from the ground, but it has not yet been clarified what type of drones they are... or what the motive is.'

The probe comes days after police said several large drones flew over Copenhagen airport, shutting the facility for hours.

Danish police this week hinted that Russia may have been behind the incident, which disrupted around 100 flights and left 20,000 passengers stranded.

Police inspector Jens Jespersen suggested the incident had all the signs of a sophisticated operation.

He said: 'The number, size, flight patterns, time over the airport. All this together indicates that it is a capable actor. Which capable actor, I do not know.'

Danish Prime Minister Mette Frederiksen called it 'the most serious attack on Danish critical infrastructure to date', adding Russian involvement could not be ruled out.

Police officers previously seen at Copenhagen Airport after all flights were diverted due to drone sightings

Your browser does not support iframes.

In further hints that they believe Vladimir Putin was behind the previously spotted drones, authorities said the perpetrator was seeking to demonstrate certain abilities.

In a social media post, Ukrainian president Volodymyr Zelensky referred to 'Russia's violation' of NATO airspace in Copenhagen. However, he did not indicate the source of his claim.

Meanwhile, Danish intelligence previously said the country is facing a 'high threat of sabotage' after the incident.

'We are facing a high threat of sabotage in Denmark. Someone may not necessarily want to attack us, but rather stress us out and see how we react,' Flemming Drejer, Director of Operations at Denmark's intelligence service PET, told a press conference this week.

Sharing further details, Jespersen said that the drones had appeared from several different directions, turning their lights on and off before finally disappearing after several hours.

Police are examining multiple theories about where the drones came from, including the possibility that they were launched from ships. Denmark's main airport sits close to a busy shipping lane used by vessels entering and leaving the Baltic Sea.

On Tuesday, Russia broke its silence to deny any involvement.

Flights at Copenhagen were either diverted or grounded for hours on Monday evening after multiple large drones were seen above the runways.

At least 35 flights were ordered to other sites, including Malmo and Gothenburg, Sweden, and Billund, Aalborg, and Aarhus, Denmark.

Authorities in Denmark diverted 31 flights to other airports, causing ripple effects that delayed or cancelled around 100 flights and affected some 20,000 passengers, a spokesperson told reporters. The airport finally reopened in the early hours of Tuesday.

Later in the evening, Oslo Airport in Norway was also forced to close and divert its planes after drones were spotted in the vicinity.

Two people were also arrested in the Norwegian capital after drones were seen at a military site.

Norwegian police detained two Singaporean citizens who were operating a drone over Akershus Fortress, which houses the headquarters of the Norwegian Armed Forces and the Ministry of Defence, according to the Norwegian state broadcaster NRK.

Drones spotted over Danish airports - fears grow Putin is testing Nato