Norwegian Frode Berg, 63, was detained in Moscow in 2017 following a sting operation by Russia's FSB security service.

Berg "is accused of spying for Norway. He gathered information on nuclear submarines," state prosecutor Milana Digayeva told RIA Novosti state news agency.

The case is classified as "totally secret" and is being heard behind closed doors, Digayeva said. Fifteen witnesses were to be called, the prosecutor added.

A former Russian police officer was accused of handing Berg files on the Russian navy and given a 13- year prison term in December.

Berg admitted to acting as a courier for the Norwegian intelligence services several times, but said he did not know the purpose of what he was delivering.

"He thought the money he was delivering had another purpose," Berg's Norwegian lawyer Brynjulf Risnes said.

Risnes, who is in Oslo awaiting permission from Russia to visit his client, told AFP that Berg "is pleading not guilty to the charges of espionage."

He said that there was a "very great risk that he will be convicted," given the lack of acquittals in Russian spy cases in recent years.

Berg risks a jail term of 10 to 20 years, the lawyer said. He hoped that if convicted, Berg would not have to serve a full sentence in Russia but could be extradited or pardoned by President Vladimir Putin.

At the end of the hearing, Berg told Norwegian journalists that "it went well, it went very well," Risnes said.

Berg's position "has not changed," his Russian lawyer Pyotr Anashkin told the TASS state news agency. "He does not accept any guilt. He's glad the trial has started and soon it will all end."

The Moscow court on Tuesday extended Berg's detention in custody until September.

Norway, a NATO member, normally enjoy good relations with neighbouring Russia, with which it shares a short land border, but these have grown more tense since the outbreak of the Ukraine crisis in 2014.