Stanford University in the United States has confirmed that one of its researchers -- a Chinese rocket scientist -- has been detained in China on charges of revealing state secrets.

The university says he was arrested in January, on his first trip home for nine years, despite securing security assurances beforehand, and months of behind-the-scenes lobbying has failed to release him.

The scientist, Hua Di, is sixty-two and reportedly in poor health.

He left China in 1989 after signing protests against the suppression of pro-democracy protests that year.

The American state department has expressed concern at the case -- and at the reported detention of about forty pro-democracy campaigners in China in recent weeks.

A spokesman said the action showed the limits to China's increased tolerance of political dissent and were a move in the wrong direction.