Four former Belarussian army officers have gone on trial in Minsk accused of military espionage for Poland.
Security officials in Belarus say the accused gathered information about a joint Belarussian-Russian air defence system, called the S-300.
The trial is being held behind closed doors as it involves state secrets.
The head of the Belarussian KGB secret service, Dmitry Vegera, announced the arrests in July. A fifth officer, a Russian, gave himself up to Russia.
In July, Belarus state TV showed two of the alleged spies apparently confessing.
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.