Authorities in China's western-most Xinjiang region have launched house-to-house searches after a clash at an Islamic school left 12 children injured, police and a rights group said Tuesday.

Xinjiang is home to around nine million mainly Muslim Uighurs, many of whom complain of religious and cultural repression by Chinese authorities -- a claim the government denies -- and the region is regularly hit by unrest.

The US-based Uyghur American Association (UAA) said police were stepping up a crackdown on underground Islamic schools after the June 6 clash in Xinjiang's Hetian city and were threatening to break down doors if locals did not cooperate.

"According to arrangements made by relevant departments at higher levels, our joint residential police bureaus will be inspecting all buildings in residential and family compounds," a police notice posted in Hetian's Gujiangbage quarter said.

"If there are families that do not open their doors, and do not cooperate with the inspection, we will force our way in, any resulting consequences will be the responsibility of the residents," photos of the notice obtained by UAA warned.

Police at the Gujiangbage bureau confirmed the searches when contacted by AFP.

"Most people are cooperating, but some people are not," one policeman told AFP. "I am not aware of any police forcefully entering homes," he said before abruptly hanging up.

According to Hetian police, 12 children were injured when "criminal suspects" ignited an incendiary bomb at the illegal Gujiangbage Islamic school as police stormed the building on June 6 to "liberate" the youths.

Three suspects were arrested, the police said at the time.

The exiled Uighur community, however, disputed the official version of events, citing sources on the ground as saying police tossed tear gas into the school.

Xinjiang has been under heavy security since July 2009, when Uighurs launched attacks on Han people -- who make up most of China's population -- in the regional capital Urumqi.

"The searches in Hotan (Hetian) are consistent with recent examples of the use of force to crack down on peaceful Uighur activities deemed outside of state control," the UAA statement said.