Beijing - Amnesty International on Thursday urged the Chinese government to reveal the whereabouts of a Mongolian human rights activist who has disappeared since he was reportedly released last week after spending 15 years in prison.

Police detained the wife and son of the activist, Hada, before his scheduled release date of December 10, the London-based human rights group and other sources reported.

Like many Mongolians, Hada, 55, uses a single name.

Photographs circulated by the New York-based Southern Mongolian Human Rights Information Centre showed Hada with his wife and son eating a meal in what appeared to be a hotel.

The information centre said a disc containing the digital pictures, which were dated December 10, was given anonymously to Hada's sister-in-law in Hohhot, the capital of China's Inner Mongolia region.

Police informed the sister-in-law Tuesday that her three relatives were detained in an unnamed five-star hotel.

Amnesty International said the police had tried last month to persuade Hada's son to 'clearly draw a line' between himself and his parents and not to take part in 'separatist activities.'

Hada's son, Uiles, was detained on suspicion of selling drugs on December 4 after rejecting the police demand, it said.

Police also raided a family bookstore in Hohhot run by Hada's wife, Xinna, and accused her of running an illegal business.

'China is using enforced disappearance to keep activists and their family members out of the spotlight while the world's attention is focused on China's first Nobel Peace Prize winner, Liu Xiaobo,' said Catherine Baber, Amnesty's Asia-Pacific deputy director.

'The Chinese authorities must immediately clarify Hada and his wife and son's current status and whereabouts,' she said. 'They cannot simply hide people they find embarrassing or inconvenient.'

Hada was sentenced to 15 years in prison in 1995 for separatism and espionage.

The charges were linked to his involvement in the Southern Mongolian Democratic Alliance, which promoted human rights, Mongolian culture and autonomy for China's Mongolians and other ethnic minorities.

About 4 million Mongolians live in China, most of them in Inner Mongolia, where they now make up less than 20 per cent of the population.

Several other prominent Chinese dissidents have disappeared or been held under house arrest in recent months after their release from prison sentences.