Belarus says it has smashed a spy ring which was passing information to Poland about a joint Belarussian-Russian air defence system, called the S-300.
The Belarus armed forces keep Soviet-era traditions alive
The head of the Belarus KGB security service, Dmitry Vegera, said four former Belarussian army officers and a Russian officer had been arrested.
Belarus state TV on Sunday showed two of them apparently confessing.
The Russian is reported to have turned himself in to the Russian security service, the FSB.
Russia's Itar-Tass news agency says the suspects face from seven to 15 years in prison if found guilty.
Polish politicians have repeatedly condemned the authoritarian rule of Belarus President Alexander Lukashenko, who has cracked down on opposition protests.
Mr Lukashenko accuses Poland of interfering in his country's internal affairs and of trying to foster popular protest against him.