A UN human rights committee has heard there are credible reports that China is holding a million Uighurs in "counter-extremism centres".
Gay McDougall, a member of the UN Committee on the Elimination of Racial Discrimination, raised the claims at a two-day UN meeting on China.
She said she was concerned by reports that Beijing had "turned the Uighur autonomous region into something that resembles a massive internment camp".
China did not immediately respond.
Its 50-strong delegation said it would address questions on Monday, when the session in Geneva continues.
Beijing has previously denied the existence of such camps.
Who are the Uighurs?
The Uighurs are a Muslim ethnic minority mostly based in China's Xinjiang province. They make up around 45% of the population there.
Xinjiang is officially designated as an autonomous region within China, like Tibet to its south.
Reports that more and more Uighurs and other Muslim minorities are being detained in Xinjiang have been circulating for some months.
What is Beijing accused of?
Human rights groups including Amnesty International and Human Rights Watch have submitted reports to the UN committee documenting claims of mass imprisonment, in camps where inmates are forced to swear loyalty to China's President Xi Jinping.
The World Uyghur Congress said in its report that detainees are held indefinitely without charge, and forced to shout Communist Party slogans.
It said they are poorly fed, and reports of torture are widespread.
Most inmates have never been charged with a crime, it is claimed, and do not receive legal representation.
China is said to carry out the detentions under the guise of combating religious extremism.
What does China say?
The Chinese government denies the existence of these camps.
In April, Laura Stone, a senior diplomat in the US State Department, said tens of thousands of people had been detained in "re-education centres" amid a government crackdown.
In response, Chinese Foreign Ministry spokeswoman Hua Chunying stated that, "everyone can see that people of all ethnicities in Xinjiang live and work in peace and contentment and enjoy peaceful and progressing lives".
The claims come on a day of worsening religious tensions elsewhere in China.
In the north-western Ningxia region, hundreds of Muslims engaged in a standoff with authorities on Friday to prevent their mosque from being demolished.
Officials said the newly-built Weizhou Grand Mosque had not been given proper building permits. However, human rights groups say there is increasing official hostility towards Muslims in China, where religious activities remain tightly controlled by the government.
Source: BBC NEWS