The UN rights chief will release a long-awaited report on alleged violations in China's Xinjiang region before leaving office later Wednesday, despite strong pressure from Beijing not to publish.

"The report on Xinjiang will go out by the end of the day," said Jeremy Laurence, a spokesman for outgoing High Commissioner for Human Rights Michelle Bachelet, who promised to publish the report before leaving office at the end of the month.

The announcement means the report will come out in the final 12 hours of her four-year term.

Amid the allegations swirling around abuses in Xinjiang, Bachelet came under increasing pressure to investigate and speak out about the far-western region of China. Beijing stands accused of detaining more than one million Uyghurs and other Muslim minorities there.

Nearly a year ago, the former Chilean president told the UN Human Rights Council an independent evaluation of the situation in Xinjiang was needed, and indicated that her office was finalising a report on the matter.

But the report was repeatedly delayed amid growing impatience from rights groups and some countries.

Speaking earlier Wednesday, Chinese foreign ministry spokesman Zhao Lijian branded the report a "farce" and hoped that Bachelet would not publish it in her final hours in office.

"We firmly oppose the release of the so-called Xinjiang-related report by the UN Human Rights Office. This report is a farce orchestrated by the US and a small number of Western powers," he said.

"We hope that the high commissioner will make the right decision."

Zhao said the UN rights chief should act with "objectivity, impartiality, non-selectivity and non-politicisation" and oppose "double standards on human rights issues".

In her farewell press conference on Thursday, Bachelet admitted she was under "tremendous pressure to publish or not to publish" the long-delayed report.

"We're trying very hard to do what I promised," Bachelet said. "But I will not publish or withhold publication due to any such pressure."

Campaigners accuse China of a litany of abuses in Xinjiang, including mass incarceration, forced labour, compulsory sterilisation and the destruction of Uyghur cultural and religious sites.

The United States and lawmakers in other Western countries have gone as far as accusing China of committing "genocide" against the minority groups.

Beijing vehemently rejects the claims, and has long insisted it is running vocational training centres in Xinjiang designed to counter extremism.

It claims the allegations are part of a plot by the United States and other Western nations to smear China and contain its rise.

In May, Bachelet concluded a rare six-day visit to China that also took her to Xinjiang.

But the long-awaited trip to the region sparked criticism from rights groups.

During her visit, she had urged Beijing to avoid "arbitrary and indiscriminate" measures in Xinjiang. But she faced criticism for her lack of firmness and for capitulating to a stage-managed tour of the region orchestrated by Beijing.

"It was an utter failure," Human Rights Watch chief Ken Roth told AFP afterwards.

"She's going to publish the report as she walks out the door, which is not ideal," he added.