The drone incursion that stopped flights at Copenhagen airport on Monday night was the most severe attack on Danish infrastructure so far, Denmark's Prime Minister Mette Frederiksen said.

Kastrup airport in Copenhagen was forced to shut for several hours from around 20:30 (18:30 GMT) on Monday following the sighting of a number of drones.

It says something about the times we live in and what we as a society must be prepared to deal with, Frederiksen told reporters.

Russian involvement could not be ruled out, Frederiksen added - although Kremlin spokesman Dmitry Peskov called the allegations unfounded.

The Danish PM made a link between last night's events in Denmark and the recent Russian drone incursions in Poland and Romania.

Frederiksen said the motive for the incursion in Copenhagen was likely to disrupt, create unrest... to see how far you can go and test the limits.

Danish intelligence mirrored this assessment, stating the country is facing a high threat of sabotage.

In nearby Norway, Oslo airport also experienced closures after possible drone sightings.

Approximately 20,000 passengers were affected by the closure of Copenhagen airport, which resumed operations after midnight local time.

Earlier on Tuesday, Danish police stated they did not know who was behind the drones, but evidence suggested it was a capable actor. Inspector Jens Jespersen noted the drones were operated by someone likely skilled and equipped.

In a social media post, Ukrainian President Volodymyr Zelensky referenced Russia's violation of Nato airspace in Copenhagen on 22 September.

The Norwegian Police Security Service is working to clarify whether the unconfirmed drone sighting at Oslo airport could be connected to the drones observed in Denmark.

Tensions have been escalating, with recent incursions and increased military presence from NATO in response to Russian activities in the region.