Russia on Saturday released a physicist who was sentenced to 13 years in prison after being convicted of spying for China despite his insistence that the information involved had long ago been declassified.

Valentin Danilov was released early from prison number 17 in the Siberian city of Krasnoyarsk with over three years of his original sentence remaining, Russian news agencies said.

Danilov was first arrested in 2001 and sentenced in 2004 to 14 years in prison -- later reduced to 13 years -- for spying for China and embezzling 466,000 rubles ($15,000) from a state university to assist his work.

The criminal case against him was opened at the start of ex-KGB agent Vladimir Putin's first Kremlin term, and for many represented a return to the paranoid hunts for traitors in Soviet times.

After his release, Danilov said he hoped to return to science and that he would move back to the research town of Akademgorodok outside Siberia's biggest city Novosibirsk to live with his wife.

He also vowed to seek to clear his name and take his case to the European Court of Human Rights.

"I have still never been told what secret it is that I possess," Danilov said, quoted by the RIA Novosti news agency.

Danilov, a former director of the thermo-physics centre at Krasnoyarsk State University, was convicted of selling state secrets to a Chinese engineering company.

In a hugely controversial legal process, he was initially acquitted by a jury in 2003, but the verdict was then quashed by the Supreme Court and he was rapidly sentenced to 14 years in jail.

The case focused on his signing in 1999 of a contract with the Chinese engineering firm to build appliances to simulate the impact of a space environment on satellites.

Danilov always insisted the research involved had been declassified by Russia after the fall of the USSR and was available in the public domain.

Several top Russian scientists have defended his cause, saying he handed over no state secret and that his work with foreigners is normal practice in post-Soviet Russia as cash-strapped scientists try to make ends meet.

Showing a wry sense of humour, Danilov said after his release he wanted to work in science but would steer clear of the space industry.

"I will work in science but not space because everything (in Russia) to do with space is always (seen as) a state secret," he said.