"We have thoroughly investigated the area, and we have not found anything," North Jutland Police Chief Christian Tilsted said.

Denmark said on Thursday it was weighing whether to invoke Article 4 of NATO's founding treaty after drone incursions briefly shut two of its airports and affected military installations in its western Jutland region overnight.

Estonia is extending a fence along its border with Russia and building anti-tank ditches and bunkers in preparation for a potential conflict with Moscow.

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.

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

These incursions, said Politico, raise “difficult questions” for the Nato alliance. Although “Moscow insists it’s done nothing wrong”, Poland has shot down some of the drones that flew over its skies, and several Nato nations are warning that “they’re ready to shoot down Russian aircraft entering their airspace”.

Two NATO officials, who asked for anonymity to discuss sensitive operational matters, told NBC News U.S. fighter jets were taking off for assignments somewhere in Eastern Europe.

NATO is preparing a defensive military response to the drone incursion in Poland to strengthen deterrence across the alliance’s eastern flank, according to a person familiar with the matter.

Ukraine should be fully integrated into plans for a "drone wall" to protect NATO's borders because it has the experience and know-how to be able to do it, Lithuania's foreign minister said on Monday.