"After hard work, those who have been educated and trained, most of them have returned to the society," Shohrat Zakir, chairman of the Xinjiang Uyghur Autonomous Region, told reporters on Tuesday.

At their height, the centres are believed to have held as many as 1.5 million ethnic minorities, mostly Muslim Uighur and Kazakhs.

Zakir defended the centres as "vocational skill education and training centres" designed to prevent the spread of Islamist extremism.

He also said the centres teach ethnic minorities language skills as well as "how to carry out normal religious activities."

Xinjiang's detention centres are one part of a greater crackdown in the province, which was sparked by deadly riots in 2009.

The crackdown has intensified since Chen Quanguo, the former Communist Party leader of Tibet, was transferred to Xinjiang in 2016.

Outside the camps, Uighurs are subject to a surveillance network that includes frequent checkpoints equipped with facial-recognition machines, security cameras placed inside people's homes and Han Chinese officials who forcefully reside with Muslim families.

Australian Associated Press