Two other drones were launched last week and are participating in the NATO mission Task Force X.

These unmanned ships, which will be tested for three months by the Danish navy, are on "surveillance missions," Kim Jorgensen, Danish National Armaments Director, told AFP.

"They will do it in the Baltic, they will do it in the North Sea, and they will do it in other Danish internal waters," he said.

"Some of our waters are quite busy, so I'm quite eager to see how this will work with an unmanned vessel."

The Baltic Sea, which can only be accessed through the Danish straits, accounts for over eight percent of global maritime traffic according to data from France's ISEMAR institute of maritime economics.

Tensions over the Baltic Sea have heightened since Russia's full-scale invasion of Ukraine in 2022.

Several undersea Baltic cables were damaged last year, with many experts calling it part of a "hybrid war" carried out by Russia against Western countries.

The unmanned ships -- called Voyagers -- are 10 meters (33 feet) long and can autonomously navigate for three months.

The sensors equipped on them can scan down to a depth of 300 meters.

They are manufactured and operated by Saildrone, a company that works with the US navy.

"These vehicles are set up to deliver maritime domain awareness. So think about your eyes and ears above and below the surface, seeing things that we previously had no insight into," Richard Jenkins, founder and CEO of Saildrone, said.

"There are many nefarious things that happen in our oceans, from smuggling -- smuggling of people, smuggling of weapons, smuggling of narcotics -- to nefarious damage, so undersea cables, undersea pipelines, and illegal fishing," he added.

"Things that we need to monitor and monitor closely, we can't currently do that," Jenkins said, noting there was not enough manned ships to cover the oceans.