The Latvian military has reported spotting a Russian submarine off the coast of Latvia on Monday, the third such sighting reported since November. While the submarine was not in Latvia’s territorial waters, it was traveling in the country’s exclusive economic zone, the 200-mile stretch from Latvia’s coast that it can use for energy production.

According to the Latvian armed forces, the submarine was only 27 miles from the start of Latvia’s territorial waters, or around 39 miles from the country’s coastline.

Previous sightings of Russian naval vessels occurred in November 2014 and February 2015 when a corvette-class warship and a submarine appeared in the exclusive economic zone.

However, unlike in territorial waters, warships of other states are allowed to use the economic zone for training and transit.

In recent months Russia has been more active in the region, with military aircraft and ships using the area for training. In some cases, aircraft have briefly crossed into the airspace of other countries, such as Estonia.

Baltic countries have become increasingly concerned about their own sovereignty since Russia annexed Crimea in March 2014, fearing that a precedent of annexation during Soviet times and their geographical location on Russia's doorstep could see them sucked into a wider regional conflict.

The war in eastern Ukraine that's concerning Baltic governments has seen more than 6,000 people die since it began one year ago. Pro-Russian rebels still occupy large parts of the Donbas region and have defeated the Ukrainian military in most battles.

Meanwhile, the Latvian government has announced plans to strengthen its eastern border with Russia by spending $15.9 million on advanced security.

"Within four years, we are planning to establish on the eastern border a border strip 12 meters wide, equipped with modern security systems,” said Latvian Interior Minister Rihards Kozlovskis. “It will improve the response time of Latvian border guards.”