The New York-based Committee to Protect Journalists (CPJ) has raised concerns over a Chinese dissident released last week after completing a 13-year sentence.

Chen Ziming, accused of helping organise the 1989 pro-democracy demonstrations in Tiananmen Square, has been released from house arrest but is still under "tight surveillance", the CPJ said, adding that it was also concerned about Mr Chen's health..

There are worries about Mr Chen's health

On Thursday, China freed a Tibetan nun imprisoned since her mid-teens for her political activities

Both releases came ahead of Chinese President Jiang Zemin's visit to US President George Bush's Texas ranch next week.

China often releases dissidents ahead of high-level meetings where the subject of human rights is likely to be raised.

Chen Ziming and a colleague, Wang Juntao, were convicted in 1991 for "counterrevolutionary" activities and sentenced to 13 years in prison.

They were accused of being the "black hands" behind the Tiananmen protests, in which the Chinese military sent in tanks to quell demonstrations by pro-democracy students, killing hundreds of protesters.

'Timely' release

Mr Chen, 50, and Mr Wang, 43, ran a think-tank that conducted pioneering social surveys and published Economics Weekly, a journal focussing on the social consequences of China's economic reforms.

Both men were released on medical parole in 1994 after pressure from then-US President Bill Clinton for China to improve its human rights record. Mr Wang now lives in New York.

Ngawang Sangdrol was in prison for 10 years (TIN)

But Mr Chen was returned to prison more than a year later after he staged a one-day hunger strike to mark the Tiananmen Square protests. The authorities said he was strong enough to serve out his sentence if he was healthy enough to fast.

In 1996, Mr Chen, who has cancer, was again released on medical parole, and has been under house arrest ever since.

A London-based monitoring group on Thursday said the release of Tibetan nun, Ngawang Sangdrol, so close to President Jiang's trip to Texas, was "timely".

"The Chinese side wants things to go smoothly," Thierry Dodin of the Tibet Information Network, told BBC News Online.

Ngawang Sangdrol, born in 1977, was convicted in 1992. She is believed to be China's longest-serving female prisoner convicted of counterrevolution

Her release came a year after a prison report reduced her sentence by 18 months, saying she was showing "genuine repentance."

In July, a Tibetan scholar, Tanak Jigme Sangpo , believed to be China's longest-serving political prisoner, was released nine years early and flew to exile in the US.