BEIJING — In the deadliest ethnic violence in China since 2009, 21 people were killed in confrontations Tuesday between police and Uighur residents of Kashgar, the country’s westernmost city.

Among the dead were 15 police and neighborhood security officers and six people that the state media described as “mobsters.’’

Kashgar, which lies close to China’s borders with Kyrgyzstan and Tajikistan, has been a frequent site of violence between the dominant ethnic Han Chinese and the Uighurs, a Muslim minority. As is often the case, it was difficult to confirm details of the incident and the account offered up by authorities was vague.

Tianshan.com, a website run by the Chinese government, reported Wednesday that three neighborhood security officers tried to confiscate knives from a family in Bachu county, on the outskirts of Kashgar. The “mobsters” were hiding in the basement of a house and ambushed the officers, taking them hostage, the website reported. When police responded to the scene, a gun battle broke out and the house was burned down.

The website said that 10 of the police officers killed were ethnic Uighurs themselves.

“A preliminary investigation showed the mobster gang were planning to launch terrorist activities,’’ the report said.

Liu Hainjun, a graduate student from the village, said in a telephone interview that government officials were told to stay in their offices during the day for fear of attacks.

There are clashes often in the area between Chinese authorities and Uighur gangs. While Chinese frequently allege that the gangs are Islamic separatists, human rights activists say that the confrontations are often simply criminal cases.

The death toll from Tuesday’s clash is the highest since 2009, when 200 people died in rioting in Urumqi, the capital of the Xinjiang region.

Uighurs, who were the majority in the region before an influx of Han Chinese settlers in recent years, complain that their culture is being eradicated and that unreasonable restrictions prevent them from practicing their religion.

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