The United Nations High Commissioner for Human Rights Michelle Bachelet arrived in China on Monday, kicking off the first trip by a holder of the office since 2005 amid concerns that it could lead to an endorsement rather than scrutiny of China's rights record.

During the six-day trip, Bachelet will visit Xinjiang, where the High Commissioner's office said last year it believes the mostly Muslim ethnic Uyghurs have been unlawfully detained, mistreated and forced to work.

The Chinese foreign ministry said it welcomes her but rejects "political manipulation" when asked by media if she can visit the detention centres, re-education camps and prisons where rights groups say Uyghurs have been mistreated.

China has repeatedly denied any mistreatment of Uyghurs.

"The purpose of the private visit is to enhance exchanges and cooperation between both sides and promote the international cause of human rights," ministry spokesman Wang Wenbin told a media briefing on Monday.

He said that Bachelet's visit will be conducted in a "closed loop", referring to a way of isolating people within a "bubble" to prevent the COVID-19 virus from potentially spreading.

This means that Bachelet will not be able to have free and spontaneous in-person meetings with anyone who has not been pre-arranged by China to be brought inside the "bubble".

Wang also said that the media will not be travelling with Bachelet because of the coronavirus pandemic.

The May 23-28 trip has been long in the making after Bachelet said in 2018 she wanted unfettered access to Xinjiang. China said the visit should not be based on a presumption of guilt.

Rights groups worry that if Bachelet does not press China hard enough, her post-trip report may not give a full picture and could be used by Beijing to justify its actions in Xinjiang.

The World Uyghur Congress urged Bachelet in a letter to ensure that her team can move freely, access all detention facilities and have unsupervised contact with Uyghurs.

"We are concerned the trip might do more harm than good. China could use it for propaganda purposes," a Congress spokesperson Zumretay Arkin told Reuters.

International scrutiny of the government's actions in Xinjiang heightened in 2018 after the United Nations said 1 million Uyghurs were held in "massive internment camps" set up for political indoctrination.

China initially denied the existence of any camps, then later admitted it had set up "vocational training centres" with dormitories where people can "voluntarily" check themselves in to learn about law, Chinese language and vocational skills.

The atheist ruling Communist Party said such centres are necessary to curb the "Three Forces" of terrorism, separatism and religious radicalism in Xinjiang, which borders Central Asia on China's north-western frontier.

Xinjiang Governor Shohrat Zakir said in 2019 all trainees had "graduated".