Russia's FSB security service detained Shonov, a Russian national, in Vladivostok in May 2023 and accused him of taking money to covertly supply U.S. diplomats with information that was potentially harmful to Russia.

The State Department said at the time that his case highlighted Russia's "blatant use of increasingly repressive laws" against its own citizens and said the allegations against Shonov were "wholly without merit".

A court in the Primorsky region in Russia's Far East confirmed in a statement on Friday that it had found Shonov guilty and had sentenced him to four years and 10 months in a penal colony.

Video of the verdict being read out, released by the court, showed Shonov listening inside a courtroom cage as the judge sentenced him.

The FSB published a video in August 2023 showing a purported confession by Shonov in which he said two senior U.S. diplomats based in Moscow whom Russia later expelled had asked him to collect information about Russia's war effort in Ukraine, its annexation of "new territories", its military mobilization and the 2024 Russian presidential election.

In the video, Shonov said he was told to gather "negative" information on these topics, to look for signs of popular protest, and to reflect these in his reports.

It was not clear whether he was speaking under duress.

Shonov was employed by the U.S. Consulate General in Vladivostok for more than 25 years until Russia in 2021 ordered the dismissal of the U.S. mission's local staff.

Relations between Moscow and Washington have plunged to their lowest level for decades over Russia's war in Ukraine.