Latvia's military said on its official Twitter page on Tuesday that a Russian naval ship was on the edge of the country's territorial waters.
The ship, identified as a Kashtan-class submarine support vessel, came as close as 28 nautical miles (52 kilometers) close to the country's naval borders, the military said on Twitter.
A day earlier Latvia's military reported detecting two Russian Kilo-class submarines at least 27 nautical miles away from Latvia's territorial waters.
Russia has escalated its naval activity on its western frontier in recent months. Earlier this week, Russia's Northern Fleet was put into full combat readiness for series of military drills, a move interpreted as a response to simultaneous military exercises held in Norway.
In December, Latvia's Defense Ministry claimed that Russian submarines had approached its naval borders more than 50 times in the past year and Russian planes flew near its airspace 200 times, Russian state news agency TASS reported.
Russia's Defense Ministry has neither denied nor confirmed those figures and insisted that its military movements are made strictly "in accordance with the existing international norms and practices," according to the TASS report.