A dissident Buddhist monk who fled Vietnam earlier this year has disappeared in neighbouring Cambodia while under the protection of the United Nations refugee agency.
The monk, Thich Tri Luc, has been missing for a week.
The authorities in Vietnam are already under pressure from several governments over the disappearance of three visiting Chinese dissidents who have been missing in Vietnam for several weeks.
The Paris-based Buddhist Information Bureau says Thich Tri Luc fled Vietnam in April this year to escape religious persecution.
It says he was given refugee status in late June by the United Nations High Commissioner for Refugees in the Cambodian capital, Phnom Penh.
Kidnap fears
The agency does not comment on individual cases but the BBC has confirmed that the monk was staying at a house in the city and was under UN protection.
The Buddhist Information Bureau says he disappeared on 25 July after leaving the house with an unidentified man.
The group says it is concerned he may have been kidnapped or forced back to Vietnam.
Such fears could have been prompted by reports that Vietnam police routinely cross the border into Cambodia to forcibly return ethnic minority people who fled repression in Vietnam's Central Highlands last year.
While they are mostly Protestants, Thich Tri Luc is a member of the Unified Buddhist Church which is banned in Vietnam.
He and other church leaders have been subject to detention and harassment by the Communist authorities over their calls for religious freedom and democracy.
In a separate case, the Vietnamese authorities have not yet released any information on three Chinese dissidents who have been missing for a month.
Two live in the United States and one in Paris. They are believed to have entered Vietnam from Cambodia in June.
Human rights groups outside Vietnam fear they have been kidnapped by Chinese police.