Rights organization Free Tibet says violence in China's Sichuan province is escalating.
The group reports that 10 Tibetan Buddhists have set themselves on fire since March to protest Chinese occupation of the region. Two Tibetans were shot by Chinese authorities in October during demonstrations against Beijing.
Heiner Bielefeldt, the U.N. special envoy on freedom of religion, said Chinese raids and surveillance in and around Tibetan monasteries in Sichuan province was unacceptable.
"Intimidation of the lay and monastic community must be avoided and the right of members of the monastic community and the wider community to freely practice their religion, should be fully respected and guaranteed by the Chinese government," he said in a statement.
The U.N. High Commissioner for Human Rights said restrictions on freedom of religion have resulted in the forced disappearance or displacement of hundreds of Tibetan monks.
Jeremy Sarkin, a special envoy on involuntary disappearances, expressed grave concern about the situation in China's far west.
"This heinous practice is not permitted under any circumstances," he said. "No exceptional circumstances whatsoever may be invoked to justify an enforced disappearance."