Chinese police were Thursday investigating eight suspects arrested in the ethnically divided Xinjiang region after clashes left 21 dead, with authorities reportedly accusing "terrorists" of ambushing police.

Local government officials said 15 police and social workers were killed in the western region on Tuesday, adding that six members of the mostly Muslim Uighur minority shot dead in the clashes were suspected of terrorist plotting.

But the Uyghur American Association, which is run by members of the minority living in the US, urged the international community to dismiss the terrorism claims, saying it was a way of justifying the use of extreme force.

Xinjiang, a region about twice the size of Turkey, is home to around nine million ethnic Uighurs, many of whom complain of religious and cultural repression by Chinese authorities.

"The case is in the phase of investigation and collecting evidence," an official in Xinjiang's regional information office, surnamed Yang, told AFP, adding that eight suspects had been arrested.

The state-run Global Times daily quoted local officials as saying "the terrorists may have set a trap", adding that knife-wielding men ambushed police after luring them to a house in Barchuk county, in the region's west.

But the Uyghur American Association said in a statement that terrorism claims should be viewed with "extreme caution" and accused local authorities of abusing terror charges to repress Uighurs, who see Xinjiang as their homeland.

"They always use such labels to justify their use of armed force," the statement quoted Dilshat Rexit, a spokesman for the World Uyghur Congress, another exile group, as saying.

While the Chinese authorities and state media often blame unrest in the region on "terrorism", some experts say the government has produced little evidence of an organised threat and that the violence stems from ethnic resentment.

After the latest violent episode US State Department spokesman Patrick Ventrell called on China to "take steps to reduce tensions and promote long-term stability in Xinjiang".

"We urge the Chinese authorities to conduct a thorough and transparent investigation of this incident and to provide all Chinese citizens -- including Uighurs -- the due-process protections to which they're entitled," he said.

But China's Foreign Ministry spokeswoman Hua Chunying Thursday accused the US of a "double standard" for not condemning the attack despite being a victim of terror itself, and called on the US to "reflect on its own problems".

Riots between 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 province 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, raising cries of "ethnic discrimination".

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.