Hada, who like many Mongolians goes by one name, should have completed his sentence last December but has still not been released.

Meanwhile, the Southern Mongolian Human Rights Information Centre said wife Xinna and son Uiles, who have protested his continued detention, were arrested in December in Hohhot city in Inner Mongolia and later charged respectively with "illegal business practices" and "drug possession".

Both have denied the charges and maintained their innocence. They have also refused to sign documents admitting to the allegations and to drop their protests in exchange for their freedom, the group said.

One of China's longest-jailed prisoners of conscience, Hada fell foul of authorities through writings in which he called for Mongol autonomy, and after organising peaceful demonstrations as head of the underground Southern Mongolian Democracy Alliance.

According to transcripts of a May 4 phone conversation with Xinna's sister Naraa posted on the group's website, police have also placed Hada's other relatives under strict surveillance, confiscating their phones and computers and warning them against talking about the case.

Police have brought relatives to visit Hada - being held separately from his family - to try to convince him to end repeated hunger strikes carried out in protest at his continued imprisonment, Naraa said.

Hada, who is in his mid-50s, is in poor health due to the hunger strikes, she added. He has also adamantly refused to acknowledge any guilt stemming from his conviction in the 1990s.

Telephone calls to Hohhot police departments went unanswered Tuesday.

"Chinese authorities have made it clear to Hada and his relatives that they will not be freed until they sign an undertaking to abandon their human rights activities," the Paris-based Reporters Without Borders (RSF) said in a statement. "Until now, they have refused to do this."

RSF strongly condemned what it called "trumped-up charges" brought against Hada's wife and son and called for the trio's "immediate and unconditional release".

Many of China's six million ethnic Mongols, who have cultural and ethnic ties with Mongolia, complain of political and cultural repression by China. Some refer to Inner Mongolia as "Southern Mongolia”.