Nearly two dozen activist groups said on Tuesday that crimes against humanity and genocide are taking place against Uighurs in China's remote Xinjiang region, where more than 1 million people are held in camps.
There was no immediate response to the allegations by the Chinese foreign ministry, contacted by Reuters. China says the camps are vocational education and training centres as part of counter-terrorism and deradicalisation measures.
The open letter signed by groups including the U.S.-based Uighur Human Rights Project and Genocide Watch calls on the United Nations Human Rights Council to launch an investigation.
"The atrocities include arbitrary detention of between 1 and 1.8 million people in internment camps, a widespread program of political indoctrination, enforced disappearances, destruction of cultural sites, forced labour, disproportionate rates of prison incarceration, and coercive birth prevention campaigns and policies," it said.
Under international law, crimes against humanity are defined as widespread and systematic, whereas the burden of proof of genocide - the intent to destroy part of a population - is more difficult to prove.
"These measures meet the threshold of acts constitutive of genocide, core international crimes under the Genocide Convention, which prohibits 'imposing measures intended to prevent births' among an ethnic or religious group," the groups said.
U.N. human rights chief Michelle Bachelet said on Monday she was discussing a possible visit to Xinjiang with Chinese authorities who are facing growing backlash over the treatment of ethnic Uighurs.
But activists voiced disappointment in her speech to the Human Rights Council, where China has never been the object of a resolution.
"Bachelet's remarks on China say nothing of substance - no word about the human costs of China's rights violations, including against Uighurs and in Hong Kong, nor the ongoing concerns about free expression, arbitrary detention and crackdowns on civil society," Sarah Brooks of the International Service for Human Rights told Reuters.
"Instead, the remarks speak volumes about the weak position of the human rights office vis-a-vis China," she said.
The U.S. on Monday shelved plans for a broad import ban on cotton and tomato products from Xinjiang while announcing narrower bans on products from five specific entities.
U.S. officials said the measures were aimed at combating China's use of forced labour by detained Uighur Muslims.