In a sign of growing tensions, Estonia jailed three men it has accused of spying for Russia Tuesday, Reuters reported. The suspects were described as smugglers by the country’s prosecutor’s office and sentenced to between two and almost five years in prison.

“The information on the trials and sentencing was released now as two of the trials were held behind closed doors, due to the nature of the crimes committed,” said Ilmar Kahro, a spokesman for the state prosecutor’s office, according to Reuters.

The three men, who range in age from 21 to 42, were accused of giving Russian security services information on Estonia’s security service, defense and border guard movements. Prison terms against the alleged low-level informants indicate increased vigilance as Baltic States continue to worry about Russia’s actions in eastern Ukraine where fighting between government forces and Russian-backed rebels has increased in recent days.

Relations between Russia and the Baltic States have been tense ever since the annexation of Crimea from Ukraine in March 2014. Estonia, a member of both the European Union and NATO, has accused Russia of aggressive action on its border with the latest accusation last week that Russia had violated its airspace, a charge the Kremlin has denied, Russian state news TASS reported.

The Baltic States have called for a greater NATO presence in the region with Estonia planning to expand its Amari air base to provide room for the alliance’s aircraft, a move Russia described as “provocative.” A report published by the Rand Corporation earlier this month found that if Russian troops invaded the Baltic States, NATO forces would be overrun within three days underscoring the heightened concerns in the region.

Estonia and Russia have previously accused each other’s citizens of acting as spies. In a high profile prisoner exchange last year, Estonian security officer Eston Kohver was exchanged for Aleksei Dressen, the Wall Street Journal reported. The exchange marked a rare instance of cooperation between the countries in recent months.