A Russian court found a Norwegian man guilty on Tuesday of gathering espionage about nuclear submarines and jailed him for 14 years in a verdict that could strain ties between Moscow and its NATO-member neighbour.

Frode Berg, a 63-year-old retired former guard on the Norwegian-Russian border, was detained in Moscow in December 2017 and tried behind closed doors this month. He pleaded not guilty to charges of espionage on behalf of Norway.

Berg will not appeal the verdict and plans to request a presidential pardon from Vladimir Putin that would see him freed, his lawyer, Ilya Novikov, said.

Berg, who was wearing a suit and was handcuffed as he watched proceedings from a glass cage, has admitted to being used as a courier for Norway's military intelligence but said he had no knowledge of the operation he took part in.

Putin last week appeared to leave the door open to the possibility that Berg could be pardoned or possibly exchanged as part of a prisoner swap.

Asked during talks with Norway's Prime Minister Erna Solberg about the possibility of a pardoning, Putin said: "We must wait for the court proceedings ... We will take a look at what we can do with this depending on the court's decision."

State prosecutors had asked the court to jail Berg for 14 years. The maximum penalty for espionage is 20 years.

Defence lawyer Novikov said ahead of the hearing that he was expecting a guilty verdict and a lengthy sentence, and that the defence was hoping diplomatic talks would now possibly lead to Berg's freedom.

"The question is how successful diplomatic efforts will be to secure his release," Novikov said.