The local government in China's restive northwestern region of Xinjiang says that a clash between authorities and assailants has left 21 people dead in what it describes as an act of terrorism.

The Xinjiang government propaganda office said in a news release Wednesday that 15 officers and local government officials were among the dead in Tuesday afternoon's clash. It said six assailants were killed on the spot and another eight were captured alive.

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The death toll was the highest in months in Xinjiang, which sees recurrent outbreaks of violence pitting members of the indigenous Turkic Muslim Uighur ethnic group against the authorities and majority ethnic Han Chinese migrants.

Rioting in July 2009 between Uighurs and Han killed nearly 200 people in Xinjiang's capital, Urumqi, and there have been sporadic attacks since, all of them quickly suppressed with overwhelming force by local paramilitary units.

Tuesday's violence began when three local officials reported a group of suspicious men armed with knives hiding inside a home in Selibuya township outside the city of Kashgar, the local government news release said.

The three were then grabbed by the men in the house and local police and officials rushing to the scene were taken by surprise and attacked, it said. Only later did armed units arrive, while those inside the house killed their three captives and set fire to the building, the report said.

The incident points to the chaotic nature of much of the Xinjiang violence, as well as problems with how authorities respond. Armed units are often stationed in larger towns and barracks and must be specially summoned by commanders before they can respond.

The release said 10 of those killed on the government side were Uighurs, three were Han, and two were from the Mongolian ethnic group. It said two other Uighurs were injured. The ethnicity of the assailants wasn't given and local police and government officials reached by phone said they had no additional information to give.

Xinjiang, a sprawling region that borders Central Asia, is home to millions of Uighurs (pronounced WEE'-gurs) who complain of heavy-handed rule by Beijing and say they have been marginalized by policies favoring Han.