The Belarussian security service -- still known as the KGB -- said in a statement that a military tribunal had found two men identified as A. Fenzelyau and Ye. Kochura guilty of treason.
The statement said the two were detained by March 2012 and that an investigation had found "incontrovertible proof of their criminal activity".
The ex-Soviet nation's KGB in July 2012 reported the arrest of a Lithuanian military intelligence agent and several informants as they traded secret information concerning Belarussian security.
The Minsk authorities in 2012 only identified the arrested Lithuanian as "citizen F".
The Interfax news agency said today that Fenzelyau -- jailed for 10 years -- moved to Lithuania after retiring from service in the Belarussian armed forces in 2002.
EU member Lithuania has been more friendly toward its isolated neighbour than most other Western countries and has advised against economic sanctions against Belarus in response to President Alexander Lukashenko's crackdown on the opposition.