Beijing (CNN) -- Clashes in the unsettled western Chinese region of Xinjiang killed 21 people, including police officers and local officials, authorities said Wednesday.
The attacks took place Tuesday afternoon after three local officials visiting homes in the area found "several suspicious persons" and knives in one house, the Xinjiang government said in a statement.
When the officials alerted their superiors of the situation, the people in the house took them hostage, the statement said. Police rushed to the scene, and in the ensuing violence, six of the assailants and 15 other people were killed, it said. Two other people were injured.
Police captured eight of the attackers, whom the government described as "mobsters." It described the violence as a "terrorist act."
Some of the knives recovered by police were more than a meter long, according to Hou Hanmin, spokesperson for the Xinjiang government. She told CNN that some of the captured assailants said under interrogation that they had watched videos "from overseas" that featured violence and acts of terrorism.
"Then they made those large, lethal knives and wanted to use them for Jihad," she said, referring to the Arabic term meaning "struggle."
"They had been training in their own house for several months. They were affected by extremism and hoped to commit themselves to Jihad."
Xinjiang has been hit by periodic bouts of unrest. The worst violence in decades took place in July 2009, when rioting between Uyghurs and Han Chinese left around 200 people dead and 1,700 injured in the regional capital, Urumqi.
Uyghurs are predominantly Muslim ethnic Turks who are linguistically, culturally and religiously distinct from China's majority Han population.
The Uyghur American Association, which advocates for Uyghur rights, said the Chinese accounts of the violence this week "should be viewed with extreme caution given the lack of available details and independent verification of Chinese sources."
The Washington-based association accused Chinese officials of regularly exploiting incidents in the region to use them as evidence of a perceived terrorist plot against the Chinese state.
"The Chinese government regularly makes terror allegations against Uyghurs without producing evidence to back up their claims or allowing international investigators to independently verify their version of events," the association said in a statement Wednesday.
Chinese authorities have cracked down on those they accuse of fomenting unrest in the region.
As is often the case, the government statement about Tuesday's clash didn't specify the ethnicity of the assailants. But it said that the 15 people killed who weren't "mobsters" comprised 10 Uyghurs, three Han, and two Mongolians.
The latest violence occurred in Bachu County, east of the city of Kashgar.
CNN's Ivan Watson and Feng Ke in Beijing contributed to this report.