Latvia’s armed forces purportedly spotted several Russian naval vessels operating in its prohibited economic zone, the nation’s press service said Monday. The incident marked at least the third time this year that Latvia claimed to have spotted Russian ships active in the area without authorization.

Two Russian ships and a submarine were detected about 5 miles from Latvia’s border, the country’s national armed forces said, Russians media agency Sputnik News reported. Military personnel also identified a Russian transport aircraft near Latvian airspace.

The incident occurred just hours after Swedish authorities claimed Russian ships had interfered with attempts to lay a network of cables as part of an energy deal between Sweden and Lithuania, reported EurActiv, a news agency that specializes in European Union affairs. Russian authorities said the ships were merely defending its right to conduct military drills in the area. Sweden and Lithuania each planned to discuss the incident with the Kremlin.  

Latvia previously claimed to have detected Russian warships in its waters in both February and April. 

Russia’s military has conducted a series of land, sea and air training exercises in recent months after Russian President Vladimir Putin vowed to modernize the nation’s armed forces. Western leaders have expressed increasing concern with Russia’s military activity given its annexation of the formerly Ukrainian territory of Crimea last March and its purported involvement in the ongoing eastern Ukraine conflict. Clashes between Ukrainian government forces and pro-Russian separatist rebels purportedly backed by the Kremlin have killed 6,000 people since early 2014.

In the face of Russia’s activity, the NATO military alliance has recently expanded its own military operations. Several NATO nations and Sweden, which is not part of the alliance, conducted anti-submarine naval drills in the North Sea on Monday to test procedures to locate and destroy underwater vessels.

"Russia has a right to be at sea, just as we do," said U.S. Navy Rear Adm. Brad Williamson, who commanded the operation, Reuters reported. “But incidents we have seen are not in line with international regulations … and that’s been a cause of concern.”