China could have more than one million people detained in what United Nations officials have described as a "massive internment camp that is shrouded in secrecy".
Many credible reports suggest that Uighurs and Muslim minorities have been forced into "political camps for indoctrination" in western Xinjiang, according to the UN Committee on the Elimination of Racial Discrimination.
Western Xinjian is an autonomous region of China, and UN officials believe Beijing could be treating ethnic minorities as "enemies of the state".
Vice-chairwoman of the committee Gay McDougall said members were "deeply concerned" by China's crackdown on the Uighurs and Turkic Muslim peoples.
"(China) has turned the Uighur autonomous region into something that resembles a massive internment camp that is shrouded in secrecy," Ms McDougall said.
Monitoring groups say the Uighurs have been targeted in a surveillance and security campaign that has sent thousands into detention and indoctrination centres.
But Ms McDougall suggested the number could be much higher.
"There are estimates that upwards of a million people are being held in so-called counter-extremism centres and another 2 million have been forced into so-called re-education camps for political and cultural indoctrination."
The startling claims come from numerous sources, according to Reuters, including activist group Chinese Human Rights Defenders, whose recent report revealed 21 per cent of arrests recorded in China were made in Xinjiang.
China's central government is believed to have restricted the freedoms of those living in the region over the years. Beijing is known to have imprisoned many prominent Uighurs, while others accused of terrorism have sought asylum abroad.
About 200 people died during ethnic riots in Xinjiang's capital, Urumqi, in 2009, and the Chinese government is thought to be using ongoing unrest as a justification for repression.
The US mission to the UN took to Twitter to condemn China over their alleged actions, saying it was "deeply troubled".
"We call on China to end their counter-productive policies and free all of those who have been arbitrarily detained," the US mission tweeted.
Almas Nizamidin, who fled Xinjiang in 2009, told the ABC earlier this year the capital Urumqi "looked like an occupation" when he returned to find his wife who had been taken from her home.
"There were lines of tanks on the streets, and a police blockhouse every 100 metres where police officers scan people's IDs and the contents of their phones," Mr Nizamidin told the ABC.
He told the ABC several policemen arrested his wife on no official charge, but she was later seven years in prison at just 25-years-old, and two months pregnant.
Beijing slammed Canberra in 2009 when the Australian Government granted prominent Uighur activist, Rebiya Kadeer, a visa in 2009.
The Chinese Government accused Kadeer — who lives in exile in America — of being a terrorist, and strongly opposed her visit.
Canberra was vehement in its response, saying they had no reason to believe she was responsible for any terror-related activity.
A Chinese delegation of about 50 officials has made no comment on the UN committee's claims, but the review hearing is set to continue on Monday.
However, Yu Jianhua, China's ambassador to the UN in Geneva, said earlier that the government hoped to achieve equality and solidarity among all ethnic groups.