U.N. human rights investigators called on China on Tuesday to halt repression of Tibetan Buddhist monks and voiced concern that many had been arrested or had disappeared during a recent crackdown on religious freedom.

Security forces, some bearing automatic rifles, have been reported as deploying in and around Kirti monastery and other monasteries in Aba prefecture in the southwestern province of Sichuan, raiding monasteries and spying on monks, they said.

Kirti monastery has about 2,500 monks. Several are said by exile groups to have set light to themselves in protest against government-imposed religious controls.

The severe restrictions on freedoms of religion, expression and association have led to hundreds of monks reportedly leaving the monastery, with many being arrested or subjected to enforced disappearance, the independent experts said in a joint statement issued in Geneva.

The whereabouts of some monks were unknown amid concerns for their fate. Amnesty International says enforced disappearances take place when someone is arrested, detained or abducted by the state or its agents, who then deny that the person is being held or conceal their whereabouts, placing them outside the protection of the law.

Jeremy Sarkin, chairman of the U.N. working group on enforced or involuntary disappearances, said: Any enforced disappearance is unacceptable and such practices are in violation of international law.

A proposed revision to the Chinese criminal procedure law that would legalise enforced disappearances was of great concern. No exceptional circumstances whatsoever may be invoked to justify such crimes, he said.

Chinese monks have been arbitrarily arrested and detained in the restive region, the U.N. working group on arbitrary detention said without giving any figure for those imprisoned.

No individual can be arrested on the ground of peacefully exercising the rights and freedoms guaranteed under international human rights law, said El Hadji Malick Sow, chairman of that group of independent experts.

Tibetan areas in Sichuan province, which neighbours the official Tibet Autonomous Region, have been unsettled by self-immolation protests by Buddhist monks and a nun in past months.

A Tibetan Buddhist monk doused himself in fuel and set himself ablaze in far western China last week, the tenth ethnic Tibetan this year to resort to the extreme form of protest, the Free Tibet advocacy group said on October 26.

A spokeswoman for U.N. High Commissioner for Human Rights Navi Pillay said on Tuesday that her office deeply regretted the tragic loss of life and urged China to soothe the unrest.

We call on authorities to do what they can to defuse the tension and prevent further cases, U.N. rights spokeswoman Ravina Shamdasani told a news briefing in Geneva.

For the Chinese government, the protests are a small but destabilising challenge to its regional policies, which it says have lifted Tibetans out of poverty and servitude.

China has ruled what it calls the Tibet Autonomous Region since Communist troops took control there in 1950. It rejects criticisms of rights groups and exiled Tibetans and has condemned the self-immolations as destructive and immoral.

(Reporting by Stephanie Nebehay, Editing by Rosalind Russell)