China has imposed a "bewildering" array of regulations on the practice of Islam by its minority Uighur population that have severely impeded religious freedom, an advocacy group warned Wednesday.

The US-based Uighur Human Rights Project, in a study based on witness interviews, said China has pursued an "unrelenting drive" of "ever-repressive measures" against the mostly Muslim people in the northwestern Xinjiang region.

The study said China has taken a more subtle approach than simply banning religious practices and instead has "progressively narrowed" the scope of legal activity through national regulations on religion imposed in 2005.

"What we found is there is a bewildering number of regulations that Uighurs face every day. There is confusion and people are not too sure what is legal and what isn't," report co-author Henryk Szadziewski told a panel discussion.

"The fact is that even customary practices are being questioned," he said on Capitol Hill.

The study said that residents of Xinjiang, which activists call East Turkestan, were not allowed to enter mosques if they were under 18 years old or employed by the government.

The report said rules were particularly strict during the holy month of Ramadan when restaurants were forced to stay open during the day, when Muslims traditionally fast, and students and government workers were pressured to eat.

Chinese authorities have imposed prison sentences for "illegal" religious practices and banned Uighurs with Islamic attire such as beards and veils from entering certain buildings including public assistance offices, it said.

The report called for China to end its array of restrictions and to allow an independent Islamic clergy, as well as to include Uighur representatives on bodies that govern religion.

China argues that it has brought development to minority regions. It has justified security measures in Xinjiang by voicing concern over "terrorism" in the wake of the September 11, 2001 attacks on the United States.

Western-based experts say that Uighurs traditionally practice a moderate brand of Islam and that Al-Qaeda links in Xinjiang have been minimal.

Violent riots involving Uighurs and members of China's Han majority in Xinjiang's capital Urumqi in 2009 killed around 200 people.