E.U. leaders say they want to build a “drone wall” along the borders with Russia and Ukraine after incursions into NATO airspace have raised alarm.

Ukraine is sharing its expertise in drone defense with European countries. President Volodymyr Zelenskyy says Ukrainian military experts are in Denmark for joint exercises.

The vessel had its tracking equipment deactivated, indicating the vessel wished to avoid detection by Danish authorities.

Denmark's defense chief has called a series of unidentified drones being spotted near airports and military installations a "hybrid attack."

Denmark's defense ministry reported more drone sightings overnight Saturday into Sunday at several of its armed forces' locations.

Denmark will impose a blanket ban on civilian drone flights ahead of a visit by European leaders to Copenhagen next week.

At least five Danish airports saw the drones overnight, including at Aalborg and Billund airports, which had to close for hours, according to the BBC. Skrydstrup air base, where at least one drone was also spotted, is home to F-35 and F-16. fighter jets.

Denmark reported to NATO allies that unspecified "state actors" were responsible for drone incursions that shut two airports, Latvia's foreign minister told Reuters on Thursday, but Danish officials said it was still unclear who was behind the incidents.

Boosting NATO's presence close to Denmark sends "a message of assurance and cohesion within the Alliance," says a spokesperson.