One of China’s best known ethnic rights activists has been charged with “separatism” in the restive far western region of Xinjiang, in the latest sign of a tightening by the authorities in a region that is expected to receive a surge in industrial investment.

Ilham Tohti, an economist and the most prominent voice for Uighurs in China, has not been seen since security forces detained him with some of his students in Beijing in January. His lawyer, Li Fangping, told the Financial Times on Wednesday that authorities in Urumqi, the capital of Xinjiang, are refusing to meet him or allow him to see his client.

Xinjiang has been the scene of several violent clashes between security forces and local Uighur groups demanding more autonomy in recent years.