Beijing: A feared smartphone app being used by Chinese police to surveil the Uighur ethnic minority has been reverse-engineered by a human rights group, revealing that even turning your own phone off can trigger suspicion.

When The Sydney Morning Herald and The Age travelled to China's western region of Xinjiang in late 2018, residents there said Muslim Uighurs regularly had their phones inspected by police. If "suspicious" content was found, they were sent to re-education centres, where hundreds of thousands of Muslims are believed to be detained.

A police officer is seen through a train window in Turpan, Xinjiang, last year. Credit:Bloomberg

Human Rights Watch contracted a German security company to assess the app that police are using on their phones when they question Uighurs on the street, at home, or apply to travel abroad.

The information gathered is then pulled into a central system used to profile residents.