China says it has arrested nearly 13,000 people it describes as terrorists in the traditionally Islamic region of Xinjiang since 2014 and broken up hundreds of "terrorist gangs".

The figures were included in a government report on the situation in the restive northwestern territory that seeks to respond to growing criticism over the internment of an estimated 1 million members of the Uighur and other predominantly Muslim ethnic groups.

China describes the camps as vocational training centres and says participation is voluntary.

Former detainees say they were held in abusive conditions, forced to renounce Islam and swear allegiance to China's ruling Communist Party.

The lengthy report issued Monday also says "law-based de-radicalisation" in Xinjiang has curbed the rise and spread of religious extremism.