China has charged a prominent ethnic Uighur professor with separatism, as security forces flooded into the far western region of Xinjiang amid continuing anti-government protests.

The prosecutor's office in the Xinjiang regional capital of Urumqi made the announcement about economics professor Ilham Tohti in a brief online statement on Wednesday.

Tohti was arrested in mid-January and later accused of promoting and supporting Xinjiang's independence from China, a red line for Beijing which views the region as an inseparable part of the country. However, he has firmly rejected accusations of fomenting separatism, which can carry penalties of several years in prison.

A brief statement on the official microblog of the Urumqi prosecutor said that Tohti's case had been turned over to a city court, the next legal step needed before he can be put on trial.

Tohti has been a vocal advocate for equal rights for the mainly Muslim Uighur ethnic minority in Xinjiang.

The US on Wednesday called for his release, saying it was "concerned about the lack of transparency concerning his welfare and access to legal representation".

The case against Tohti is the latest sign of the government's hardening stance on dissent in Xinjiang, where unrest in the past year or so has killed more than 200, including several police, Reuters quoted Chinese state media as saying.

On Monday, the government said a gang armed with knives and axes attacked a police station and government offices in the town of Elixku in Shache County, 200km away from the old Silk Road city of Kashgar in China's far west.

Security forces killed dozens of people in response.

A witness in Kashgar told Reuters news agency that a long convoy of armed police vehicles, including at least five armoured personnel carriers, drove into the city.

Source: Agencies