The man, identified as Tun Sheniyun, was arrested in Moscow on 28 October last year but his arrest was kept secret until now.

He allegedly sought technological and repair documentation on the Soviet-era S300 surface-to-air system.

The case surprised analysts, as Russia has already sold the system to China.

Mr Tun is thought to have been acting on the instructions of the Chinese state security ministry, Russia's Federal Security Service (FSB) said.

Formally, he worked as an interpreter for official delegations, it added.

There was no immediate reaction to news from Chinese officials.

Fifteen S300 units were exported to China last year, Reuters news agency notes.

China is also licensed to manufacture the system locally.

In Russia itself, the S300 has been replaced with a newer system, the S400.

The espionage charge carries a prison sentence of between 10 and 20 years, AFP news agency reports.

Analysts told the agency they found the case mystifying because of both the delay in its announcement and the weapons system involved.

Furthermore, Russian Prime Minister Vladimir Putin is due to visit China next week on a trip designed to strengthen ties.

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, correspondents say.