The United Nations released a bombshell report into serious human rights abuses in China's Xinjiang region late Wednesday, saying torture allegations were credible and citing possible crimes against humanity.

The report, in the making for around a year, was released in Geneva at 11:47 pm (2147 GMT) on Wednesday -- just 13 minutes before Michelle Bachelet's four-year term as UN High Commissioner for Human Rights expired.

The former Chilean president was determined to ensure the report saw the light of day -- despite intense pressure from an infuriated Beijing.

"I said that I would publish it before my mandate ended and I have," Bachelet said in an email sent to AFP.

"The issues are serious -- and I raised them with high-level national and regional authorities in the country."

China has been accused for years of detaining more than one million Uyghurs and other Muslim minorities in the far-western Xinjiang region.

Campaigners have accused China of a litany of abuses, while Beijing has vehemently rejected the claims, insisting it is running vocational training centres in Xinjiang designed to counter extremism.

Bachelet eventually decided that a full assessment was needed of the situation inside the Xinjiang Uyghur Autonomous Region (XUAR).

"Serious human rights violations have been committed in XUAR in the context of the government's application of counter-terrorism and counter-'extremism' strategies," the report said.

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.

"The extent of arbitrary and discriminatory detention of members of Uyghur and other predominantly Muslim groups, pursuant to law and policy, in context of restrictions and deprivation more generally of fundamental rights enjoyed individually and collectively, may constitute international crimes, in particular crimes against humanity," it added.

The report urged Beijing, the UN and the world at large to focus its gaze on the situation described in Xinjiang.

"The human rights situation in XUAR also requires urgent attention by the government, the United Nations intergovernmental bodies and human rights system, as well as the international community more broadly," it said.

The 49-page report made no reference to genocide: one of the key allegations made by China's critics, including the United States and lawmakers in other Western countries.

Speaking on Wednesday after Bachelet's office had announced it would release the report, Zhang Jun, China's ambassador to the UN in New York, said Beijing had told her that it was "firmly opposed" to the rights assessment.

"The so-called Xinjiang issue is a completely fabricated lie out of political motivations and its purpose is definitely to undermine China's stability and to obstruct China's development," Zhang told reporters.

He said Bachelet should have stayed "independent" and not caved in to "political pressure" from Western countries.

Bachelet and her office have repeatedly explained that one reason the report was delayed was because it had been sent to Beijing first for comments, as is common with such reports.

Zhang however maintained that China had not seen the report, and was "completely opposed" to it.

"It simply undermines the cooperation between the UN and a member state. It completely interferes in China's internal affairs," he said.

Bachelet insisted that dialogue with Beijing did not mean "condoning, overlooking or turning a blind eye".

But she added: "The politicisation of these serious human rights issues by some states did not help. 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."