Xinjiang, a Chinese region the size of Alaska, has become one of the world’s most heavily policed places. Bloomberg reported in January on government experiments with facial recognition systems that alert authorities when targeted people venture more than 300 meters (1,000 feet) beyond designated "safe areas."

Local governments have also ordered residents to install satellite-tracking systems in their cars, while people must submit to facial scans to enter markets, buy fuel or visit places such as the capital Urumqi’s main bus terminal.

China’s President Xi Jinping said the country should "strike first" against Islamist extremism after deadly attacks involving Uighurs in 2013 and 2014, as well as reports of some fighting in Syria.

Asked about the re-education camps in a regular briefing on July 27, China’s Foreign Ministry spokesman Geng Shuang said: "The Chinese government fully respects people’s religious freedom, and we guarantee that in accordance with law. All ethnic groups and the people in all regions in China enjoy full freedom of religious belief."