MOSCOW/OSLO (Reuters) - The trial of a Norwegian man suspected of espionage began on Tuesday in Moscow, in a case that has raised tensions between Russia and its NATO-member neighbor.

A general view of Moscow City Court building, ahead of a hearing for Frode Berg of Norway suspected of espionage, in Moscow, Russia April 2, 2019. REUTERS/Maxim Shemetov

Frode Berg, a retired former guard on the Norwegian-Russian border, is accused by Russian authorities of gathering information about Russian nuclear submarines on behalf of Norway.

“Berg was enlisted by the Norwegian intelligence service and agreed to work with them in return for financial reward,” Russia’s RIA news agency cited state prosecutor Milana Digaeva as saying.

“He was detained while receiving information from a Russian citizen who worked in a defense enterprise and was acting under the control of the FSB (Russia’s domestic intelligence service),” she added.

The trial, which is to be held behind closed doors, is planned to last until Thursday, said Berg’s Norwegian lawyer, Brynjulf Risnes.

“He (Berg) has all along been pleading not guilty, which he will continue to do,” Risnes told Reuters on Tuesday.

The trial began with the court examining case documents and will continue with the cross-examination of witnesses, Berg’s Russian lawyer Ilya Novikov told Reuters.

Berg has admitted to being a courier for Norway’s military intelligence but said he had little knowledge of the operation he took part in.

Norwegian daily Dagbladet reported that on his arrest Berg was carrying 3,000 euros ($3,700) in cash, and that Russian authorities suspected the money would be mailed to a Russian man to pay for information about the country’s nuclear submarine fleet.

The maximum penalty for espionage in Russia is 20 years in prison.

“Looking back historically, the chance for acquittal is quite slim,” Risnes said, adding that the defense was hoping for as short a sentence as possible and for Berg to be allowed to serve it in Norway.