"The Russian FSB arrested a Russian citizen in Kaliningrad... who for a long time had worked with the special services of Lithuania," a spokesman for the federal security service told AFP.
The suspect, who was deputy head of the prison sentencing board for a district in the Russian Baltic region, was caught with electronic devices "containing state secrets", the FSB said.
The official, identified as Lieutenant Colonel Vasily Khitryuk, was accused by the FSB of gathering military secrets from former colleagues serving in the army and other security structures.
Kaliningrad is located on the Baltic coast and separated from Russia proper by neighbouring Lithuania, as well as two other ex-Soviet republics, Belarus and Latvia. To the south, Kaliningrad borders Poland.
Relations are frequently strained between Russia and Lithuania, which was occupied for nearly five decades after World War II by the Soviet Union, only regaining independence in 1991. Lithuania became a member of both the NATO military alliance and the European Union in 2004, to Moscow's annoyance.
Asked to comment on the allegations of espionage in Kaliningrad, Lithuanian Prime Minister Gediminas Kirkilas was quoted by the Baltic News Service (BNS) news agency as saying: "We do not spy there."
Kirkilas also stressed that Lithuania strives to have good relations with all of its neighbours, including Russia.
Earlier this month, Lithuania expelled the first secretary of the Russian embassy in Vilnius, reportedly because he was involved in "illegal intelligence activities", BNS said.
Lithuania in 2004 ordered six Russian diplomats, including Moscow's military attaché and his deputy, to leave the country, accusing them of spying. Moscow retaliated by expelling four Lithuanian diplomats, including the Baltic state's military attaché, and later refused to accredit his replacement.

Copyright AFP (Agence France-Presse), 2006