"Berg is accused of spying for Norway. He collected information about nuclear submarines. He was recruited by the Norwegian intelligence service and agreed to cooperate with it for a monetary reward. He was detained while receiving information from a Russian citizen who worked at a defence company and acted under FSB supervision," state prosecutor Milana Digaeva said.
Retired Norwegian border inspector Frode Berg, 62, was arrested in Moscow in December 2017.
According to his attorneys, Berg, who is facing a minimum of six years in prison but can be sentenced to as long as 20 years, denies being a spy.
The trial is closed to the public.