A Norwegian man has been found guilty in Moscow of spying on Russian nuclear submarines and sentenced to 14 years in a high-security jail

Frode Berg, 63, who used to work as a guard on the Norwegian-Russian border, admitted acting as a courier for his country’s intelligence services but denied spying.

The verdict could strain relations between Moscow and its Nato-member neighbour, which have deteriorated in the past five years.

In a trial behind closed doors, prosecutors said Berg was caught with documents he had received from an employee of a military facility who was shadowed by Russian intelligence.

The former guard, who was handcuffed and held in a glass cage in court, said he had no idea he was involved in espionage. No more details of the hearing have been revealed as they are classified.

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Berg will not appeal against the verdict but will seek a pardon from president Vladimir Putin, according to his lawyer, Ilya Novikov.

He said the case had been a set-up, and that his client expected his government to “undertake diplomatic efforts”, warning that he was facing “basically a life sentence”.

Norway’s foreign ministry said it was working to bring Berg home.

Last week, Mr Putin did not rule out the possibility that Berg could be pardoned or part of a prisoner swap.

During talks with Norwegian prime minister Erna Solberg, he 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.”

For years, Berg had been a well-known figure in the Russian-Norwegian Arctic border area, taking an active role in cultural and humanitarian exchange projects.

For decades relations between the two countries were amicable but they have worsened since 2014, when Russia annexed Ukraine’s Crimea peninsula.

Additional reporting by agencies