Frode Berg, 63, was detained in Moscow in 2017 following a sting operation by Russia's FSB security service.

A former Russian police officer was accused of handing Berg files on the Russian navy and given a 13-year prison term in December.

Berg has admitted to acting several times as a courier for the Norwegian intelligence services but said he thought he was only carrying money.

The verdict was read out in front of journalists in the Moscow court room on Tuesday, the only time the case was open to the media.

Berg's lawyer Ilya Novikov said after the sentencing that his client was not planning to appeal.

"Berg has no illusions, he accepted (the verdict) calmly. He expects his government to undertake diplomatic efforts," Novikov said. "We see no practical use in appealing."

- 'Basically a life sentence' -

He said before the verdict that an effort had probably been made to gather intelligence but that Berg was unaware of it.

"He's been used without his knowledge," Novikov said. "We cannot talk about gathering any secret information."

He warned that Berg might not survive a lengthy prison sentence in Russia.

"He's 63 and in the conditions of Russian prisons that's basically a life sentence."

Novikov said he hoped diplomatic efforts could persuade Russian President Vladimir Putin to sign a presidential pardon so Berg could be released and go home.

The case was raised during a meeting between Putin and Norwegian Prime Minister Erna Solberg in Saint Petersburg last week.

Asked about the possibility of Berg being pardoned, Putin said: "A person can only be pardoned if he has been convicted. He has not been convicted yet."

Novikov said that the fact that Putin spoke publicly about the case with the Norwegian prime minister was "probably a good sign" for his client.

Foreigners convicted of espionage have "a chance for their case to be resolved on some political level", the lawyer said, though he added: "We know nothing of what is going on behind the scenes."

NATO member Norway normally enjoys good relations with neighbouring Russia, with which it shares a short land border.

But relations have grown more tense since 2014, when Russia annexed Crimea and a pro-Russian insurgency erupted in eastern Ukraine.

In recent years Russia has broadened its definition of treason and espionage, alarming rights groups. Espionage is punishable in Russia by up to 20 years in prison.

In another high-profile case, former US Marine Paul Whelan was detained in late December and charged with espionage.

His lawyer said Whelan was handed a USB drive containing state secrets but he had thought it was information about Russian culture.