From a distance they look almost like ordinary sailboats, their sails emblazoned with the red-and-white flag of Denmark.

But these 10-meter (30-foot) -long vessels carry no crew and are designed for surveillance.

Four uncrewed robotic sailboats, known as “Voyagers,” have been put into service by Denmark’s armed forces for a three-month operational trial.

Built by Alameda, California-based company Saildrone, the vessels will patrol Danish and NATO waters in the Baltic and North Seas, where maritime tensions and suspected sabotage have escalated sharply since Russia’s full-scale invasion of Ukraine on Feb. 24, 2022.