United Nations’ human rights experts voiced alarm on Thursday over alleged Chinese political re-education camps for Muslim Uyghurs and they called for the immediate release of those detained on the “pretext of countering terrorism.”

The UN Committee on the Elimination of Racial Discrimination cited estimates that up to one million Uyghurs may be held involuntarily in extralegal detention in China’s far western Xinjiang province.

Its findings were issued after a two-day review of China’s record, the first since 2009, earlier this month.

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China’s foreign ministry rejected the allegations at the time, saying that anti-China forces were behind the criticism of Beijing’s policies in Xinjiang. It has never officially confirmed the existence of detention centres there.

China has said that Xinjiang faces a serious threat from Islamist militants and separatists who plot attacks and stir up tensions between the mostly Muslim Uyghur minority and the ethnic Han Chinese majority.

But the panel decried China’s “broad definition of terrorism and vague references to extremism and unclear definition of separatism in Chinese legislation.” This could be used against those peacefully exercising their rights and facilitate “criminal profiling” of ethnic and religious minorities, including Uyghurs, Buddhist Tibetans and Mongolians, it said.

In its conclusions, the panel said it was alarmed by: “Numerous reports of detention of large numbers of ethnic Uyghurs and other Muslim minorities held incommunicado and often for long periods, without being charged or tried, under the pretext of countering terrorism and religious extremism.”

“We are recommending to China if this practice exists, to halt it. We are asking China to release people if they don’t have a legal ground to be detained,” panel member Nicolas Marugan told Reuters Television.

Chinese officials were not immediately available for comment on the panel’s criticisms on Thursday.

The independent experts regretted that there were no official numbers on people detained “for even non-threatening expressions of Muslim ethno-religious culture like daily greetings.”

During the review the experts said they had received many credible reports that around a million Uyghurs are held in what resembles a “massive internment camp that is shrouded in secrecy.” Panel member Gay McDougall described it as a “no-rights zone.”

The panel expressed concern over reports of “mass surveillance disproportionately targeting ethnic Uyghurs,” including through frequent police checks and scanning of mobile phones at checkpoints.

It also cited reports alleging that many Uyghurs who had left China had been forced to return to the country, and it called on Beijing to disclose their whereabouts and status.

Ms. McDougall cited allegations that more than 100 Uyghur students who returned to China from countries including Egypt and Turkey had been detained, with some dying in custody.

A bipartisan group of U.S. lawmakers on Wednesday urged Washington to impose sanctions on Chinese officials responsible for rights abuses of Muslims in Xinjiang, saying the region was being turned into a “high-tech police state.”

The UN panel urged China to allow Tibetans access to passports for foreign travel and to promote the use of the Tibetan language in education, the judicial system, and media.

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“The reports that we have received say that Tibetan is not on an equal footing with Chinese Mandarin in Tibet,” Mr. Marugan said, adding that Tibetans had the right to speak their own language and for it to be preserved.

The panel asked China to report back within a year on its main concerns.