Rights groups condemned China on Wednesday for charging a prominent Uighur critic of government policy towards the mostly-Muslim minority with separatism -- which can carry the death penalty. Ilham Tohti, an economics lecturer at a university in Beijing, has been formally accused of the offence, his wife Guzaili Nu'er told AFP. Tohti has been one of the most prominent critics of Chinese policy in Xinjiang, the vast western region where most Uighurs live and which is periodically hit by violent clashes between locals and China's security forces. China maintains that unrest in the region is caused by terrorist groups seeking an independent state, an account denied by Uighur rights groups who complain of widespread religious repression and economic discrimination. Rights group Amnesty International urged people to send letters to Chinese leaders demanding Tohti's immediate release. "He is a prisoner of conscience, detained and arrested solely for the peaceful exercise of his right to freedom of expression," the group said on its website. Dilshat Rexit, a spokesman for the World Uyghur Congress, an exile group, said that "China's accusations of separatism are merely an excuse for suppression of those with different political opinions". "We call on the international community to monitor China and free this Uighur scholar," he added in a statement. The US-based Uyghur Human Rights Project said in a statement that the charge against Tohti "reflects not only a zero tolerance policy to Uighur dissent, but also the growing intractability of China towards international criticism of its ethnic policies". The group also called on China to account for the whereabouts of Tohti, who has not been allowed to contact his family or see a lawyer since his arrest last month. Li Fangping, a lawyer appointed by Tohti's family, told AFP the scholar was being held at a facility in Xinjiang's capital Urumqi, adding he had lodged a formal request to meet his client and was awaiting a response. Beijing's foreign ministry spokeswoman Hua Chunying told reporters: "China's judicial authorities will deal with the relevant case in accordance with law since China is a country under the rule of law." Tensions in Xinjiang, a strategically important region which abuts central Asia, have risen in the past year with a series of deadly clashes. Police also blamed suspects from the region for apparently deliberately crashing a car in Beijing's central Tiananmen Square in October, killing two tourists and the three people in the vehicle. The incident led President Xi Jinping to call for a security push in Xinjiang. Tohti had challenged the government's account of the crash. The United States and the European Union have both condemned Tohti's arrest. China's courts are tightly controlled by the ruling Communist party, and have previously handed down lengthy jail sentences to intellectuals who have spoken out against the authorities.