BEIJING, April 27 (AFP) - A US-based rights group has called for the release of an ethnic Mongolian activist arrested by Chinese authorities as he was about to board a plane to the United States to attend a UN meeting.

Sodmongol, who like many Mongolians goes by one name, had cleared customs in Beijing on April 18 and was waiting for his flight when he was taken away, the US-based Southern Mongolian Human Rights Information Center (SMHRIC) said.

"We still do not know where he is being held and how his situation is," Enhebatu Togochog, head of the rights group, told AFP in an email.

"Currently we are working with the United Nations Office of the High Commissioner of Human Rights and the UN Voluntary Fund for Indigenous Populations to urge the Chinese authorities to release him immediately."

The 45-year-old Sodmongol, who campaigns for the rights of ethnic Mongolians in China, was due to attend a UN meeting on indigenous issues in New York, the rights group said in a statement on its website.

Many of China's roughly six million ethnic Mongolians complain of political and cultural repression -- a claim denied by Beijing, which says it protects the rights and cultures of all of its 56 ethnic groups.

Activists have said traditional herders are being forced off their grasslands, and an influx of Han immigrants has swept Mongolian culture aside in once strongly Mongolian areas.

Ethnic Mongolians also complain they have been forgotten by a world focussed on troubles in China's restive Tibet and Xinjiang regions.

According to Togochog, Sodmongol had been under scrutiny before his planned departure.

"He sent me an email on April 17 and told me that the Chinese State Security Bureau personnel were looking for him through his wife and his colleagues," Togochog said.

"He said, 'If you don't see me at Newark Airport, that means I'm arrested.'"

Sodmongol's wife told the rights group that police had raided the couple's home in the northeastern province of Liaoning last week and informed her of her husband's arrest.

The group said she was reluctant to talk, apparently under pressure from Chinese authorities.

Police in Beijing were not immediately available for comment, and police in the city of Chaoyang where Sodmongol lives said they were not aware of the issue.