Major airports in two European countries have been forced to shut for hours due to unauthorised drones flying through their airspace.

Several unidentified drones shut down airspace over Copenhagen Airport on Monday night, (early today AEST) prompting concerns that Russia could be behind the flyover above Scandinavia’s largest airport.

According to a spokesperson, the aircraft were still in the area three hours later.

The airport reopened shortly after midnight but travellers to and from the Danish capital were warned of lingering delays more than 13 hours later.

While it wasn’t immediately clear who was behind the flyover, Denmark’s prime minister and NATO’s secretary-general said that Russian involvement couldn’t be ruled out.

Danish Prime Minister Mette Frederiksen called it “the most serious attack on Danish critical infrastructure to date”.

NATO Secretary-General Mark Rutte said that it was “too early to say” whether Russia was involved, while Kremlin spokesman Dmitry Peskov rejected suggestions that Moscow could be involved.

“Every time we hear unfounded accusations,” he said in a call on Tuesday with reporters, adding that “a party that takes a serious and responsible position mustn’t make such unfounded accusations again and again”.

Oslo Airport was also shut down due to a drone sighting.

Police in the Norwegian capital said they had arrested two foreign nationals for flying drones over a restricted area. 

There was no indication that the incident was connected to the events at the airport or in Copenhagen.

Norwegian media reported the drones were flown over the Akershus Fortress, a mediaeval castle that is sometimes used for government events.

In Copenhagen, police said the drones came from several different directions, turning their lights on and off, before eventually disappearing after several hours.

“We have concluded that this was what we would call a capable operator,” Danish police Chief Superintendent Jens Jespersen told reporters.

“It’s an actor who has the capabilities, the will and the tools to show off in this way,” he added.

Jespersen said no suspects had been identified yet.

The incidents come as Europe remains on high alert after a series of Russian incursions into NATO airspace in recent weeks.

NATO shot down a number of Kremlin drones that had entered Polish territory two weeks ago, before intercepting three Russian jets in Estonian airspace over the weekend.

In an emergency meeting of the United Nations Security Council in New York, British Foreign Secretary Yvette Cooper said Moscow was risking “a direct armed confrontation between NATO and Russia” with its violations.

“Our alliance is defensive, but be under no illusion … if we need to confront planes that are operating in NATO airspace without permission, then we will do so,” she said.

New US ambassador to the UN Michael Waltz added that the United States and its allies “will defend every inch of NATO territory”.

Addressing the Russian delegation directly, Polish Deputy Prime Minister Radosław Sikorski said: “We know you don’t care for international law and you are incapable of living in peace with your neighbours”.

“Your insane nationalism contains a lust for domination that will not cease until you realise that the age of empires is over and that your empire will not be rebuilt…

“If another missile or aircraft enters our space without permission – deliberately or ‘by mistake’ – and gets shot down, and the wreckage falls on NATO territory, please don’t come here to whine about it. You have been warned.”

Jespersen said authorities had not been able to rule out the possibility of the drones over Copenhagen Airport being part of a Russian hybrid attack.

Investigators are looking at how the drones reached the airport – whether it was by land or possibly on boats coming through the strategic straits into the Baltic Sea.

– with Associated Press, CNN