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

UN 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 late last night.

She had visited China in May.

The lengthy report said 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 assessment raised concerns about the treatment of people held in China's so-called "Vocational Education and Training Centres".

"Allegations of patterns of torture or ill-treatment, including forced medical treatment and adverse conditions of detention, are credible, as are allegations of individual incidents of sexual and gender-based violence," the report said.

Michelle Bachelet released the report just 13 minutes before her four-year term
as UN High Commissioner for Human Rights expired

"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 UN report said.

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

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

The Chinese flag behind wiring at a housing compound in Yangisar, western Xinjiang

Read more: 5 key allegations in UN's Xinjiang report

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 UN 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," Mr Zhang said.

"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.

It was reported last month that China had asked Ms Bachelet to bury the report, according to a Chinese letter that was confirmed by diplomats.

Ms Bachelet confirmed last week having received the letter which she said was signed by about 40 other states, adding her office would not respond to such pressure.

Ms Bachelet plans to return to Chile to retire. Many candidates have applied for the job but no successor has yet been named by Secretary-General Antonio Guterres whose choice must then be approved by the General Assembly in New York.

"Frankly to issue the report as she's walking out the door minimises the report," Kenneth Roth at Human Rights Watch said.

"By issuing and running she is giving up, she's not doing anything with it, (she is) just kind of dropping it into the bin and leaving the office."