Mysterious drone sightings across Denmark since September 22 have prompted the closure of several airports, with Denmark hinting at possible Russian involvement, though Moscow denies the charge.

Drones were observed over Danish military sites Saturday night for the second straight day, Denmark's army said on Sunday.

Copenhagen is to host an EU summit on Wednesday and Thursday.

"Denmark will host EU leaders in the coming week, where we will have extra focus on security. Therefore, from Monday to Friday, we will close the Danish airspace to all civilian drone flights," Transport Minister Thomas Danielsen said in a statement.

"In this way, we remove the risk that enemy drones can be confused with legal drones and vice versa," he added.

A violation of the ban can result in a fine or imprisonment for up to two years, the ministry said.

Justice Minister Peter Hummelgaard said in the same statement the purpose of the ban was to simplify the work of police and other authorities.

"The police are on heightened alert, and our authorities must use their forces where necessary to take care of Danes and our guests."

He said the ban would mean police would not have to "spend their efforts on civilian drones" that did not pose a problem to security and police.

Danish police said Saturday they had received more than 500 reports of drone flights from the public, most of which were dismissed as being not of interest.

In neighbouring Norway, airport operator Avinor said late Sunday that drone "activity" was observed at Bronnoysund Airport, one of four airports in northern Norway where air and land restrictions for drones had been extended to 10 kilometres (6.2 miles) from Saturday until Monday as "a precautionary measure".

"This affected one incoming flight, which was instructed to delay its landing and has now diverted to an alternate airport," Avinor said in a statement sent to AFP.

Avinor provided no details about the number of drones observed or where they may have come from.

- NATO 'enhances vigilance' -

Norway is already investigating "possible sightings of drones" on Saturday near its biggest military base, Orland, where its F-35 fighter jets are parked.

And Germany said Saturday it wants to authorise its military to shoot down drones after a "swarm" was spotted over the northern state of Schleswig-Holstein, which borders Denmark.

NATO said it had "enhanced vigilance" in the Baltic following the intrusions.

The reinforced measures "include multiple intelligence, surveillance and reconnaissance platforms and at least one air-defence frigate" in the region to the west of Russia, alliance spokesman Martin O'Donnell told reporters.

The string of drone sightings in the past week comes on the heels of drone incursions in Polish and Romanian territory and the violation of Estonian airspace by Russian fighter jets, which raised tensions in light of Russia's ongoing invasion of Ukraine.

Danish investigators have so far failed to identify those responsible for the drone flights over Denmark. But Prime Minister Mette Frederiksen said this week that "there is one main country that poses a threat to Europe's security, and it is Russia".

Moscow said Thursday it "firmly rejects" any suggestion that it was involved in the Danish incidents.

Defence ministers from around 10 EU countries agreed Friday to make a so-called "drone wall" a priority for the bloc.