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, as well as the violation of Estonian airspace by Russian fighter jets.

Frederiksen said that the motive for the incursion in Copenhagen had likely been to disrupt, create unrest... to see how far you can go and test the limits. Danish intelligence mirrored this assessment, saying the country was facing a high threat of sabotage.

Someone may not necessarily want to attack us, but rather stress us out and see how we react, said Flemming Drejer, director of operations at Denmark's intelligence service PET.

In nearby Norway, Oslo airport too was closed for a period after possible drone sightings. Around 20,000 passengers were impacted 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. A number of large drones which had come from different directions quite a long way away had been observed at Copenhagen airport.

Police did not shoot down the drones because the airport is located in a densely built-up area and because there were planes in the air. In a post on social media, Ukrainian President Volodymyr Zelensky referenced Russia's violation of Nato airspace in Copenhagen on 22 September.

Tensions have been escalating recently after Russian drones and aircraft ventured into central and eastern European airspace, three and a half years after Moscow launched its full-scale invasion of Ukraine.