Frode Berg, 64, a retired Norwegian border guard, was detained in Moscow in 2017 following a sting operation by Russia's FSB security service and freed on Friday as part of an exchange reminiscent of the Cold War.
Prosecutors accused him of spying on the country's nuclear submarines and a Russian court sentenced him to 14 years imprisonment in April.
“I was convicted of espionage but I'm not a spy,” Berg told a press conference in Oslo.
“Call me naive if you wish, but I have the feeling I was deceived by E-tjenesten,” he said, referring to Norway's intelligence service.
Berg said at the request of an acquaintance, who he believed was linked to Norway's secret services, he agreed to make a “delivery” in 2014 to an address in Moscow.
He said he acted as a courier four other times, without knowing what he was carrying.
Berg said he tried to stop delivering the letters twice but felt “pressured” by his acquaintance.
“I am disappointed and bitter about what happened,” he added.
Berg was freed on Friday as part of an agreement between Lithuania and Russia that resulted in him and two Lithuanians being released in exchange for two Russian spies being held in Vilnius.
Norwegian authorities have declined to comment on the possible links between Berg and the intelligence service.
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