China has arrested “more suspects” over violent clashes in the western Chinese region of Xinjiang that killed 21 people, state media is reporting.

State-run  news agency Xinhua said 11 more suspects had been detained on Monday. Eight suspects were already in custody after violence on April 23, when 15 police and community workers and six “terrorists” were killed in a gunfight.

Police also found homemade explosives, weapons and flags, and vowed an “iron-handed crackdown against terrorism,” senior security official Meng Hongwei said, according to Xinhua.

Beijing blamed the attack on the East Turkestan Islamic Movement, a “violent terrorist group” who were caught making explosives, triggering the deadly clashes last week, reports BBC.

Meng did not say whether the suspects were ethnic Uighurs or whether they were members of the separatist group.

Rights groups say the charge is being used to justify the authorities’ use of force against members of the mostly Muslim Uighur minority, who have accused Beijing of religious and cultural repression in Xinjiang, according to AFP.

“The claims of terrorism are suspected of being an excuse to oppress Uighurs,” Dilshat Rexit, a spokesman for the World Uighur Congress, said.

The US rejected calls by China to condemn “terror attacks” in the ethnically divided region and instead urged Beijing to “provide all Chinese citizens, including Uighurs,” their human rights.