Photo/Illutration Uyghur security personnel patrol near the Id Kah Mosque in Kashgar in western China's Xinjiang region, Nov. 4, 2017 (AP Photo)

China's "arbitrary and discriminatory detention" of Uyghurs and other Muslims in its Xinjiang region may constitute crimes against humanity, the outgoing U.N. human rights chief said in a long-awaited report on Wednesday.

U.N. High Commissioner for Human Rights Michelle Bachelet, who has faced criticism from some diplomats and rights groups for being too soft on China, released the report just minutes before her four-year term ended. She visited China in May.

The U.N. Human Rights Office said in its 48-page report that "serious human rights violations have been committed" in Xinjiang "in the context of the government's application of counter-terrorism and counter-'extremism' strategies".

"The extent of arbitrary and discriminatory detention of members of Uyghur and other predominantly Muslim groups ... may constitute international crimes, in particular crimes against humanity," the U.N. office said.

She recommended the Chinese government to take prompt steps to release all those detained in training centers, prisons or detention facilities.

"There are credible indications of violations of reproductive rights through the coercive enforcement of family planning policies since 2017," the office said.

It added that a lack of government data "makes it difficult to draw conclusions on the full extent of current enforcement of these policies and associated violations of reproductive rights."

Rights groups accuse Beijing of abuses against Uyghurs, a mainly Muslim ethnic minority that numbers around 10 million in the western region of Xinjiang, including the mass use of forced labour in internment camps. The United States has accused China of genocide.

China has vigorously denied any abuses in Xinjiang and issued a 131-page response to the U.N. report, which Beijing's mission in Geneva described as a "farce" planned by the United States, Western nations and anti-China forces based on false information and the assumption of guilt.

Speaking ahead of the report's release, China's ambassador to the United Nations in New York, Zhang Jun, said Beijing had repeatedly voiced opposition to it. He said the U.N. human rights chief should not interfere in China's internal affairs.

"We all know, so well, that the so-called Xinjiang issue is a completely fabricated lie out of political motivations and its purpose definitely is to undermine China's stability and to obstruct China's development," Zhang told reporters on Wednesday.

"We do not think it will produce any good to anyone, it simply undermines the cooperation between the United Nations and a member state," he said.

Bachelet said her report took "considerable work and review" and she wanted to deal with input from the Chinese government last week in the spirit of constructive dialogue.

"Dialogue and engagement is about trying to build trust – incrementally - even when it seems unlikely. My own experience in Chile showed me the value of this approach," she said.

"To be perfectly honest, the politicization of these serious human rights issues by some States did not help," she added. "They made the task more difficult, they made the engagement more difficult and they made the trust-building and the ability to really have an impact on the ground more difficult."