The move came after China announced targeted sanctions against two Americans, a Canadian and a rights advocacy body who had denounced Beijing's treatment of them and other minorities.
The UN is in negotiations with Beijing for a visit "without restrictions" to Xinjiang to see how the Uighur minority is being treated, Secretary-General Antonio Guterres has said in an interview broadcast.
At least one million Uighurs and people from other mostly Muslim groups have been held in camps in the northwestern region, according to US and Australian rights groups, which accuse Chinese authorities of forcibly sterilising women and imposing forced labour.
The Chinese government denies any mistreatment and has said that people of all ethnic groups in the Xinjiang Uighur Autonomous Region and the Tibetan region enjoy wide-ranging freedoms.
The UN's High Commissioner for Human Rights Michelle Bachelet said last month that reports of arbitrary detention, ill-treatment, sexual violence and forced labour in Xinjiang necessitated a thorough and independent assessment.
In February, Bachelet said talks to organise a visit had begun but no agreement has yet been reached.
Bachelet's visit "is being negotiated at the present moment between the office of the High Commissioner and the Chinese authorities," Guterres said in an interview aired on the Canadian Broadcasting Corp's Rosemary Barton Live show.
"And I hope that they will reach an agreement soon and that the human rights commissioner will be able to visit China without restrictions or limitations," he added.
"A serious negotiation is at the present moment taking place between the Office of the (UN Human Rights) Commissioner and the Chinese authorities," Guterres told Canada's CBC television network.
"I hope that they will reach an agreement soon" to allow a visit "without restrictions or limitations," he added.
Guterres said the Chinese had repeatedly affirmed to him "that they want that mission to take place."
Sanctions against Americans
On Saturday, Beijing announced sanctions against two Americans, a Canadian and a rights advocacy body that had criticised its alleged treatment of the Uighurs, which US officials have said constitutes genocide .
Guterres said he was also following "with concern" the fate of two Canadians, Michael Kovrig and Michael Spavor, who are being held prisoner in China on charges of espionage.
Their detention, which Ottawa has denounced as "arbitrary," is widely viewed in the West as a reprisal for the arrest and continued detention in Canada of Meng Wanzhou, an executive of Chinese telecommunications giant Huawei.
"Our position has been very clear," Guterres told CBC: "that in all situations of this kind, there must be due process and full respect for the human rights of the people involved."