BEIJING — China's most prominent ethnic Mongol dissident has been reunited with his family after completing a 15-year prison term, a rights group said Monday, but authorities had yet to confirm his release.

Hada, who like many Mongols goes by one name, on Friday completed a sentence handed down in the 1990s for espionage and separatism charges after he advocated greater freedoms for China's six million Mongols.

But authorities in China's Inner Mongolia region remain mum on whether he is free, and earlier this month his fellow activist wife Xinna and their son Uiles disappeared into police custody ahead of the release.

Pictures have since been anonymously posted on the overseas human rights news website Boxun.com showing a significantly aged Hada and his family reunited and sharing a meal.

The photos, bearing a December 10 timestamp, show the three sitting on a couch with several dishes of food spread out on a table before them.

The US-based Southern Mongolian Human Rights Information Center (SMHRIC) quoted a sister of Xinna as saying the same pictures were delivered to her by a police official.

"These pictures seem to be pretty recent and authentic, but the three are still not set free," the sister, Naraa, was quoted saying by the group.

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

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

The Mongol rights centre said Naraa and other relatives had received no confirmation on the status and whereabouts of Hada and his family.

Calls to the Mongol culture bookstore operated by Xinna in the Inner Mongolia capital Hohhot went unanswered, as did calls to her cell phone.

Rights groups said the bookstore, long a centre of Mongol dissent, was sealed up when Xinna was taken away by police on December 4 and that the steps showed authorities were worried Hada's release could incite his supporters.