China has issued a protest over remarks the U.S. ambassador-at-large for international religious freedom made criticizing Beijing's polices toward Muslim and Tibetan Buddhist minorities.

The foreign ministry's office in Hong Kong said a speech Sam Brownback gave Saturday "slandered" China's religious policies. It said it had registered its dissatisfaction with the U.S. Consulate in the semi-autonomous Chinese region, where the speech was given.

U.S. officials and UN experts say China is believed to be holding 1 million Uighurs, Muslims and members of other majority Muslim ethnic groups in political education camps in Xinjiang.

The government says those camps are vocational training centers designed to rid the region of extremism.

Brownback said China's government "is at war with faith," and called for the release of those being held over religion.