Russia has detained a French citizen for illegally entering the country from Estonia and opened a criminal investigation, the Federal Security Service (FSB) said Tuesday.

EU and NATO member Estonia shares a 300-kilometre long (185 miles) border with Russia and tensions between the countries have increased since the beginning of Moscow's large-scale military intervention in Ukraine last year.

"The foreigner admitted that he illegally crossed the state border of the Russian Federation, bypassing the established checkpoints from Estonia to Russia by boat across the Narva River," the FSB said in a statement.

"It was established that the individual is a French citizen," the FSB said, according to the RIA Novosti state-run news agency.

News agencies citing the FSB reported that the individual was detained for up to 30 days pending trial, and that a criminal case had been opened.

They reported that charges of illegally crossing the Russian border carry a maximum penalty of six years behind bars.