China on Monday vowed it would "never be soft" in its crackdown in Xinjiang, after a massive leak of government documents shed new light on the mass detention of Muslims in the far-west region.

Over 400 pages of internal papers obtained by the New York Times showed that President Xi Jinping ordered officials to act with "absolutely no mercy" against separatism and extremism in Xinjiang.

The documents, leaked by an unnamed official, included unpublished speeches by Xi as well as directives on the surveillance and control of the Uighur minority, the newspaper reported on Saturday.

China's foreign ministry spokesman Geng Shuang on Monday accused the Times of "turning a blind eye to the facts" while "taking meaning out of context to publicise so-called internal documents, slander and smear counter-terrorism and de-radicalisation efforts in Xinjiang".

"China will never be soft in its fight against violent terrorists," Geng said at a regular press briefing.

Human rights groups and outside experts say more than one million Uighurs and other mostly Muslim minorities have been rounded up in a network of internment camps across the fractious region.

China, after initially denying the camps existed, describes them as vocational schools aimed at dampening the allure of Islamist extremism and violence through education and job training.

China has been accused of rounding up more than a million Uighurs and mostly Muslim minorities in Xinjiang
China has been accused of rounding up more than a million Uighurs and mostly Muslim minorities in Xinjiang AFP / WANG Zhao

Geng said that "thousands of violent terrorist incidents occurred in Xinjiang" between the 1990s and 2016, but current policies had prevented any attacks in the past three years.

"The leaked papers clearly reveal and confirm what has been going on in Xinjiang with regard to the all-out suppression and control of the non-Han peoples there (and elsewhere in China), including the massive detention and forced education camps," Fei-Ling Wang, a professor at the Georgia Institute of Technology, told AFP.

At the same time, Wang said the leak showed the Chinese Communist Party is "not nearly as monolithic as the Party pretends".

The Times said the papers were leaked by a member of the Chinese political establishment who requested anonymity and expressed hope that the disclosure would prevent the leadership, including Xi, from "escaping culpability" for the mass detentions.

The documents included the confession of one local official, Wang Yongzhi, who was punished for releasing more than 7,000 people from camps in Xinjiang.

Wang feared that "rounding up so many people would knowingly fan conflict and deepen resentment", the Times reported.

Geng said at his briefing Monday that "officials and members of all ethnic groups in Xinjiang fully support" the government's policies in the region.