News of the detention of suspected undercover Chinese spy by Russian security service is unclear why it has been released now. It comes less than a week before Prime Minister Vladimir Putin visits China as part of efforts to strengthen ties with Beijing.

Chinese citizen Tun Sheniyun had been accused of trying to buy sensitive material on a Russian anti-aircraft missile system while working under the guise of a translator for official delegations, Russia's Federal Security Service (FSB) said, according to the state-run RIA news agency.

"An investigation revealed that a Chinese citizen on assignment from the Ministry of State Security of the People's Republic of China was working as a translator for official delegations and was attempting to gather ... documents on the anti-aircraft S-300 missile system from Russian citizens in exchange for money," RIA quoted an FSB statement as saying.

It said the suspect was detained on 28 October last year. Prosecutors filed charges of attempted espionage with the Moscow City Court on Tuesday, said the FSB, the main successor of the Soviet KGB.

The relationship between the two neighbouring powers has seen highs and lows since their 1950s Cold War alliance.

Although Beijing's once heavy dependence on Russia for defence has waned, analysts say the Chinese have copied aspects of some of the higher-end Russian military technology, including parts of the new T-50 stealth fighter.

The FSB's statement said the suspected spy was trying to buy technical and maintenance documents for the surface-to-air missile system, of which China bought 15 units last year.

Analysts say clients often complain that Russia, the second largest weapons exporter in the world, fails to follow up on technical maintenance for the arms it sells.