China has arrested 11 more suspects over clashes in the ethnically divided region of Xinjiang last week that left 21 people dead, state media said Monday.

China blamed the violence in the western Chinese region on "terrorists", but rights groups say the charge is used to justify the authorities' use of force against members of the mostly Muslim Uighur minority.

State-run news agency Xinhua said all 19 suspects had now been arrested after the incident on April 23 in Kashgar's Bachu County, which saw gunfights break out that left 15 police and community workers and six "terrorists" dead.

Xinhua, citing Xinjiang police, said the suspects were from a "terrorist group" that was founded in September 2012 and the deadly clashes broke out when they were caught making explosives.

The report said they watched terrorist video clips, had tested explosive devices, and planned to "do something big" in the densely populated areas of Kashgar in the summer.

Officers arrested eight members of the group on the day of the clashes.

Senior security official Meng Hongwei said police had found homemade explosives, weapons and flags, and vowed an "iron-handed crackdown against terrorism", according to Xinhua.

Many Uighurs complain of religious and cultural repression by Chinese authorities, which rights groups identify as the source of unrest in Xinjiang.

"The claims of terrorism are suspected of being an excuse to oppress Uighurs," Dilshat Rexit, a spokesman for the World Uyghur Congress which is run by exiled members of the minority, said in an email.

He called on Chinese authorities to publish an independent investigation into their accusations of terrorism, echoing an earlier call by the United States which was dismissed by China as evidence of a "double standard".

Local authorities in Xinjiang have attempted to stop females from wearing full-veiled dress, and prevent males from wearing long beards, notices posted on local government websites showed.

"The practice of minority women and children wearing Arab clothing, and the wearing of full beards, will disappear," said one notice on the website of Yining, a city in Xinjiang.

Violent riots involving Uighurs and members of China's Han ethnic majority in Xinjiang's capital Urumqi in 2009 killed around 200 people, leading the ruling Communist Party to tighten surveillance and boost investment in the region.

The region saw more than half of China's "endangering state security" trials last year, but is home to less than two percent of the country's population, suggesting "ethnic discrimination", the Dui Hua Foundation advocacy group said.

According to official figures, 46 percent of Xinjiang's population is Uighur, while another 39 percent are Han Chinese, after millions moved to the area in recent decades.