The United States on Wednesday urged China to free a prominent Uighur academic who faces charges that could carry the death penalty after criticizing government policy.

The US was "deeply concerned" for Ilham Tohti, an economics lecturer in Beijing who was charged following more than a month of detention, said State Department spokeswoman Jen Psaki.

"We call on Chinese authorities to release Mr. Tohti and to guarantee him the protections and freedoms to which he's entitled under China's international human rights commitments, including the freedom of expression," Psaki told reporters in Washington.

Tohti has emerged as one of the most visible critics of China's policy toward the Uighurs, a mostly Muslim ethnic group concentrated in the western Xinjiang region, where tensions have mounted in recent years.

He has warned against stigmatizing Uighurs after a series of violent incidents.

Tohti, whose initial arrest was also condemned by the United States and European Union, was formally accused of separatism, a charge that could carry the death penalty.

Amnesty International described Tohti as a prisoner of conscience and exiled Uighur groups warned of a darkening climate for the minority group.

The US-based Uighur 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."

Li Fangping, a lawyer appointed by Tohti's family, told AFP the scholar was being held in Xinjiang's capital Urumqi and that he was waiting for a response to his request to see him.

Chinese foreign ministry spokeswoman Hua Chunying told reporters in Beijing: "China's judicial authorities will deal with the relevant case in accordance with law since China is a country under the rule of law."