Human Rights Watch and more than a dozen other human rights and civil society organisations including Amnesty International and the World Uighur Congress, have published an appeal for the UN's human rights body to send an international fact-finding mission to the region.

"The abuse in Xinjiang today is so severe that it cries out for international action," Human Rights Watch chief Kenneth Roth told reporters in Geneva.

Up to a million Uighurs and members of other mostly Muslim minority groups are being held in extrajudicial detention in camps in Xinjiang, according to a group of experts cited by the UN.

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Dolkun Isa of the World Uighur Congress told reporters via video link from Oslo that his group believes the numbers detained in Xinjiang had reached "maybe three million".

Beijing says the "vocational education centres" help people steer clear of terrorism and allow them to be reintegrated into society.

But critics say China is seeking to assimilate Xinjiang's minority population and suppress religious and cultural practices that conflict with Communist ideology and the dominant Han culture.

In their statement, the organisations insisted that the sites were in fact "political education" camps, where people are sent "for their perceived disloyalty to the government and the Chinese Communist Party."

Reports of torture, suicides

"In these camps, they are subjected to forced political indoctrination, renunciation of their faith, mistreatment, and in some cases torture," the statement said.

"There have been reports of deaths inside the facilities including suicides of those unable to bear the mistreatment," Amnesty International head Kumi Naidoo said in a video statement.

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HRW's Roth lamented "the still muted global outcry."

"China gets away with this extraordinarily brazen abuse because first, few people know who the Uighurs are," he said, adding that the lack of outcry was also linked to the fact that "China has shown itself willing to use its economic clout to suppress criticism."

Roth pressed UN Secretary-General Antonio Guterres "to say something".

"He has been notably silent on one of the most important, .. the most brazen human rights abuses, .. because he is worried about upsetting the Chinese," Roth said.

UN rights chief Michelle Bachelet has addressed the situation, voicing concern last September before the Human Rights Council at "deeply disturbing allegations of large-scale arbitrary detentions of Uighurs and other Muslim commnities in so-called reeduction camps across Xinjiang."

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She has said her office is seeking access to Xinjiang to check reports about the camps, with Beijing insisting UN officials can come only if they agree to stay out of the country's internal affairs.

In their statement, the NGOs insisted that "an international fact-finding mission mandated by the Human Rights Council is needed to uncover the truth."

They stressed that China's membership in the 47-seat Human Rights Council "does not exempt it from scrutiny in the face of serious allegations."

The reports of "sweeping violations", they said, "require immediate and urgent attention."