Rights group wants information about fate of those caught up in crackdown on Turkic Muslim ethnic group.
Amnesty International has called on China to end its campaign of "systematic repression" against Uighur Muslims and demanded information about what has happened to those in detention.
Organisations including the UN and Humans Rights Watch (HRW) say that China is holding Uighurs in what it calls "re-education camps" but those with relatives inside call "concentration camps".
Multiple testimonies by Uighurs who have fled China back up the reports of mass imprisonment of the Turkic ethnic group.
Inside, prisoners are allegedly forced to reject Islamic practices and anything seen as contrary to China's dominant Han culture.
"The Chinese government must not be allowed to continue this vicious campaign against ethnic minorities in northwest China," said Nicholas Bequelin, Amnesty International's East Asia Director.
"Governments across the world must hold the Chinese authorities to account for the nightmare unfolding in the XUAR [Xinjiang Uyghur Autonomous Region].
"Hundreds of thousands of families have been torn apart by this massive crackdown."
For its report, Amnesty interviewed more than 100 people whose relatives in their native Xinjiang province had gone missing and were believed to be imprisoned in the camps.
According to the organisation, those who resisted or failed to show enough progress inside the camps were subject to beatings and other forms of torture.
"The mass detention camps are places of brainwashing, torture and punishment," Bequelin said.
The Uighurs are subject to repressive measures inside China, such as heavy surveillance, restrictions on movement, and prohibition of religious expression.
Amnesty also said the country was recruiting spies in overseas Uighur communities, adding to the pressure many of those who fled feel.
China rejects the accusations of systemic repression of the Uighurs, but does not allow independent monitoring of the camps by journalists or rights groups.
SOURCE: AL JAZEERA NEWS