Drones have been seen across the Scandinavian country, including over military sites, since September 22, prompting brief closures at several airports and a ban on all civilian drone flights until Friday.

The drone intrusions were "extremely well-executed and also clearly designed to humiliate the authorities, just to show that you can actually fly drones over Danish airports and critical infrastructure", Rasmus Dahlberg, an expert on hybrid threats, told AFP.

Police have not been able to identify the perpetrators but Danish government officials have pointed the finger at Russia.

Danes are "worried" and "there is a big need to explain" why this could be happening to Denmark, Dahlberg said.

Prime Minister Mette Frederiksen reiterated Thursday in a video clip posted on social media that only one country "poses a threat to Europe's security -- and that's Russia".

"We have been very vocal and very aggressive vis-a-vis Russia," Peter Viggo Jakobsen, an associate professor at the Royal Danish Defence College, told AFP, recalling Denmark's strong support for Ukraine since the start of Russia's full-scale invasion in 2022.

"And of course that annoys Russia. So in that sense, it's obvious that Denmark has been targeted," he said.

- 'Easy victim' -

According to data compiled by the Kiel Institute, Denmark allocates 2.89 percent of its GDP to bilateral aid to Ukraine, making it the biggest contributor in proportion to its wealth.

"The support exposes us to hybrid attacks like this," Dahlberg said, adding that Denmark's intelligence service had noted as much in a risk assessment.

Denmark announced in early September that it would become the first country to host a Ukrainian weapons production facility for long-range missile fuel near the Skrydstrup air base.

But Denmark's support for Ukraine has obscured its shortcomings when it comes to its own rearmament.

The government was "sort of hoping that by spending a lot on Ukraine, people would forget that they didn't spend on their own defence," Jakobsen said.

"That's the game we've been playing in Denmark and that's why our drone defences are relatively poor."

"We are an easy victim," he said.

The first mystery drone flights also began just days after Denmark announced it would acquire its own long-range precision weapons for the first time, as Russia would pose a threat "for years to come".

- EU summit -

Copenhagen will host an EU summit gathering more than 40 heads of government, starting on Wednesday.

"The timing (of the drone flights) has made the situation particularly serious," Swedish Prime Minister Ulf Kristersson told reporters.

In order to ensure security at the summit, Denmark has banned civilian drone flights across the country until Friday.

"It's a draconian move. It shows how important it is for the government that nothing goes wrong, but it's not a sustainable solution," said Dahlberg.

Finland, France, Germany, the Netherlands, Norway, Poland and Sweden have all sent anti-drone technology to Denmark ahead of the summit.

Ukraine also said Tuesday it was sending a team of drone specialists to Denmark for exercises.

"Few countries could deal with such a threat alone at this point," stressed analyst Alexander With of the Danish Institute for Military Operations.

The European Union said it was satisfied with the measures Copenhagen had taken ahead of the summit.

"They have our full, full trust," a spokesman told AFP.

For Jakobsen, only one actor has the capacity to plan and execute an operation of such scope and complexity as the drone flights over Denmark.

"You need to be a state actor... How many states can you think of that would have an interest in doing this? I can only come up with one -- and that's Russia."

Moscow has said it "firmly rejects" any involvement.

Russia is also accused of being behind drone incursions in Polish airspace on September 10, when Poland and NATO allies scrambled jets.

Moscow has also denied that incident.

The European Union has announced plans to reinforce its security with a "drone wall".