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.
The spokesperson's office at the Chinese foreign ministry said it had not seen the letter, but rejected such allegations as "sensational topics concocted by some anti-China forces, a farcical attempt to smear and discredit China."
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China equally protects the legitimate rights and interests of ethnic minorities, it said. China has said the camps are vocational education and training centres as part of counter-terrorism and deradicalisation measures.
"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 labor, 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.
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UN human rights chief Michelle Bachelet said on Monday she was discussing a possible visit to Xinjiang with Chinese authorities.
But activists voiced disappointment in her speech to the Human Rights Council.
"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 United States 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.