MOSCOW (AFP) - A Moscow court has jailed an ex-policeman for 15 years for high treason after he was caught placing documents under a fake rock, allegedly for the CIA, in the latest such case in Russia.

Roman Ushakov was found guilty and sentenced Thursday for trying to transmit coded documents from the Russian interior ministry to the United States' Central Intelligence Agency, Russian news agencies reported.

Prosecutor Viktor Antipov, said the former police officer from the Siberian city of Krasnoyarsk was caught red-handed placing the documents which had been "requested" by the CIA under a fake rock.

He said investigators had found 37,000 euros (S$55,328) in cash and a thank-you letter from the CIA under the rock.

The case is the latest in a string of espionage-related revelations in Russia, most of them linked to Ukraine where government forces have been battling Moscow-backed separatist rebels for months.

Nine people have been arrested for high treason in the Russian capital alone since the beginning of 2014, the Moscow City Court was quoted as saying by Russian news agencies.

Observers say the cases have been trumped up by the authorities in what is seen as a fresh effort to clamp down on dissent. "This is absolute absurdity," special services analyst Irina Borogan told AFP, adding that Ushakov had likely been set up by Russia's federal security agency FSB - the successor to the Soviet-era KGB.

"There is nothing in Krasnoyarsk that could interest the CIA," she said, adding the FSB had probably intercepted a call he had made to the American embassy and put in place an elaborate plan to trick the former police officer and land him in jail.

Rights groups have expressed alarm over broadened definitions of treason and espionage that were put in place when Putin returned to the Kremlin for a third term in 2012.

Nearly anyone, including those without access to state secrets, could fall afoul of the vague legislation.

Mother of seven Svetlana Davydova was arrested on espionage charges in January for allegedly calling the Ukrainian embassy with information on Russian troops movements, and became a cause celebre for the country's embattled civil society.

After tens of thousands signed a petition calling on Putin to release the young mother, Davydova was released from prison, but the charges against her remain in place.

"There have been many absurd cases since the 2000s, but now the climate of fear around spies has worsened due to the conflict in Ukraine," said Borogan.