The United Nations has requested direct access to "re-education camps" in China's Xinjiang province, where more than one million Muslim majority Uyghurs have reportedly been imprisoned.
UN Human Rights High Commissioner Michelle Bachelet said on Wednesday that she wanted to verify "worrying reports" the organization had received.
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Former detainees claim they were tortured and forced to learn Chinese Communist Party propaganda at the massive camps.
Beijing says the camps are voluntary vocational training centers, designed to stamp out extremist tendencies among the Uyghur population.
However Bachelet said the UN had offered technical assistance in dealing with violent extremism, adding: "We wish to engage China in a serious dialogue on this pressing matter."
The call came a day after Barbel Kofler, the German Commissioner for Human Rights Policy, said she was refused permission to visit the camps during a trip to China.
"I am shocked by reports of the treatment of the Turkic Uyghur minority ... I would have liked to have gained a first-hand impression of the situation there and will continue to push for permission to visit Xinjiang soon," she said in a statement Tuesday.
Kofler added that she was "deeply concerned" by human rights in China.
"This is particularly affecting the critical voices in civil society -- human rights lawyers, journalists and bloggers," she said.
At a UN hearing on China's human rights record in November, more than a dozen countries called on Beijing to end their "arbitrary detention" of Uyghurs in Xinjiang.
The Chinese government has repeatedly defended its human rights record, saying quality of life has risen dramatically over the past four decades.
But the UN's Bachelet said Wednesday that human rights "are not divisible."
"You cannot prioritize, you cannot say these human rights are important and the other ones not ... You cannot invite somebody to your home, feed them, but don't allow them to speak," she said.
"You have to ensure that all human rights are protected."