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 a press conference overnight Saturday to Sunday.

O'Donnell said NATO's top brass "have been in constant contact with Danish leaders" about the drone incidents, the latest in a series of violations of the airspace of the alliance's member countries.

Mysterious drone sightings across Denmark and Norway since Monday have prompted the closure of several airports, with Denmark hinting at possible Russian involvement.

Several NATO countries say Russian fighter jets and drones have violated their airspace in Europe over recent weeks, accusing Moscow of testing the alliance.

But Moscow on Thursday rejected any suggestion it was behind the drones, with Foreign Minister Sergey Lavrov on Saturday warning NATO would risk a "decisive response" if it showed "any aggression against my country".