A Moscow court convicted a Norwegian man on Tuesday of spying on Russian navy submarines and sentenced him to 14 years in a hard labor camp. The ruling could likely strain an already fragile relationship between the Kremlin and its NATO-member neighbor.

Frode Berg, a retired former guard on the Norwegian-Russian border, was arrested in Moscow in December 2017. The 63-year-old man pleaded not guilty to charges of espionage was tried behind closed doors earlier this month.

Prosecutor Milana Digayeva told Russian news agencies on Tuesday that the Norwegian man caught red-handed with documents he had received from an employee of a military facility who was being followed by Russian intelligence.

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Berg admitted to acting as a courier for Norwegian intelligence but claims he had little knowledge about the actual mission. The Russian ex-policeman accused of passing Berg navy files was also arrested.

Berg's lawyer, Ilya Novikov, told the AFP news agency that his client had "been used without his knowledge.'

In this Monday, Oct. 1, 2018 file photo, Norwegian national Frode stands in a cage in Lefortovo district court in Moscow, Russia. (AP)

Novikov said Berg would not appeal the verdict but would seek a pardon from President Vladimir Putin.

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"We see no practical use in appealing," Novikov said. He added that Berg "expects his government to undertake diplomatic efforts."

The Norwegian foreign ministry said it had "noted" the verdict but declined to comment on its merit but added it was looking for ways to bring Berg back to Norway.

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“Norwegian authorities... wish to see the safe return of Frode Berg to Norway,” a ministry spokeswoman said.

Norway shares an Arctic border with Russia. Their relationship had been amicable for decades but took a turn in 2014 when Russia annexed Ukraine's Crimea peninsula.