Twelve children were hurt in a clash at an Islamic school in China's restive Xinjiang region Wednesday, police and an exile group said amid an escalating crackdown on "illegal" religious activities.

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.

Police in Xinjiang's Hetian city said they descended on an "illegal religious study centre" to free children being held there when criminal suspects "ignited a flammable explosive device".

"The police fought to put out the fire. All 54 children were rescued, while 12 children suffering from burns were immediately sent to hospital for emergency treatment," police said in a statement on their website.

"At present, three criminal suspects have been arrested, two of whom are injured. Three police have been injured."

The exiled World Uyghur Congress (WUC) group, however, disputed the government version of events, citing sources on the ground as saying police tossed tear gas into the school.

"Armed Chinese personnel caused a confrontation... when they suppressed a local Koran scripture study centre," spokesman Dilxat Raxit said in a statement.

"The authorities used tear gas against the children, leading to injuries on both sides," including children.

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.

Wednesday's clash comes after the WUC accused police in Xinjiang's Korla city on Monday of beating a 12-year-old boy to death after a raid on an illegal Islamic school there, charges that authorities have denied.

The child, identified as Mirazhid, 12, was detained on May 20 by police while studying the Koran at a private Islamic school, the WUC said.

China restricts religious education for its youth, forcing Muslims to seek spiritual instruction in private, unsanctioned schools, the group said.

Authorities have denied the child died in police custody and have ordered the arrest of anyone spreading "distorted facts" concerning the case, Korla police said in a statement.

The police said the death of the boy was related to a beating he received at the "illegal religious school".

Police have arrested at least one Uighur youth and sentenced him to 15 days administrative detention for circulating on the Internet "false reports" that the boy died in police custody, the police statement said.

"China safeguards the rights of its citizens to the freedom of religious faith, but the government will crackdown on the illegal study of scriptures and illegal religious meetings," the Global Times quoted a Xinjiang official as saying when commenting on the boy's death.

The potentially explosive incidents come as nine Uighurs were jailed in Xinjiang's Kashgar city for inciting separatism and disturbing public order, a Kashgar court and the WUC said.

The men were sentenced to between seven and 10 years on various charges, including harbouring "extremist religious thoughts" and holding "underground religious meetings", the WUC said.

Raxit denounced the verdicts as part of increasing political persecution of Xinjiang's largely Muslim ethnic minority.

"The court verdicts were reached without any fundamental legal procedures and were a result of the political needs of China," Raxit said in a statement.

An official at the Kasghar intermediate court confirmed the May 31 sentencings to AFP but refused to provide details.

Xinjiang is home to around nine million Uighurs, but the number of Han living there has increased dramatically over the past decade.