UN human rights investigators have urged China to halt a security crackdown on Tibetan monks in the country's south-west, warning that the measures are counter-productive and accusing officials of severe restrictions on freedoms of religion, expression and association.

The UN intervention follows a spate of self-immolations in Sichuan province, where nine monks and former monks as well as one nun have set fire to themselves since March. The Kirti monastery and others in Aba county have been under heavy security since the first case.

Beijing has issued increasingly angry responses to criticism over the issue. On Tuesday, state media invoked the 1993 siege of the Branch Davidian at Waco, Texas, in a piece attacking the Dalai Lama, the exiled Tibetan spiritual leader. Officials previously accused the Tibetan spiritual leader of "terrorism in disguise", saying that by holding prayers for the dead clergy he had incited further suicides.

In their joint statement, the UN's independent experts said: "The heavy security measures adopted have resulted in increased tensions between the Chinese authorities and members of the monastic community … 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."

Heiner Bielefeldt, the UN special rapporteur on freedom of religion or belief, said the Chinese government should fully respect the right of clergy and the lay community to fully practise their faith.

Citing reports of armed officers, security raids and surveillance, he added: "Such restrictive measures not only curtail the right to freedom of religion or belief, but further exacerbate the existing tensions, and are counter-productive."

The special rapporteurs on the rights to freedom of expression, and of peaceful assembly and association, also raised concerns, as did the chair of the UN working group on arbitrary detention.

Jeremy Sarkin, who chairs the working group on enforced or involuntary disappearances, said: "Any enforced disappearance is unacceptable and such practices are in violation of international law … This heinous practice is not permitted under any circumstances."

He said he was concerned that a proposed revision to the Chinese criminal procedure law would legalise enforced disappearances.

Separately, Nepalese police detained more than 100 Tibetan exiles who had gathered to pray for the dead clergy on Tuesday.

Around 400 people had gathered at a Tibetan Refugee Centre in Kathmandu. They began protesting on the streets after police in riot gear entered and pulled down a banner showing the Dalai Lama.

The Nepalese government has said it cannot permit demonstrations against friendly nations.