European governments are on alert after Russian incursions into NATO airspace, but the Danish authorities said they could not say who was behind the latest incidents.

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

Denmark is probing potential Russian involvement after enduring the second drone attack this week as authorities are bracing for the possibility of more incidents. Large professional drones spotted at several airports and military air bases in Denmark disrupted flights and sparked a major police operation overnight.

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

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.

Denmark's Aalborg airport, used for commercial and military flights, was closed due to drones in its airspace, police said early on Thursday, two days after the country's main Copenhagen airport was shut over drone sightings that raised European security concerns.

Drones flew over Aalborg Airport in northern Denmark overnight Wednesday into Thursday, as well as three smaller airports in the country, but it was not immediately clear who was behind the incidents,

The alliance is rethinking how it responds to nontraditional threats after drones recently appeared in the skies over Poland and Denmark.