A Tibetan teenager set himself on fire in southwest China and died on Saturday, the sensitive anniversary of the Dalai Lama's flight into exile in 1959, a rights group and US broadcaster said.

The 18-year-old monk was from the restive Kirti Monastery and self-immolated in Aba town in Sichuan province, London-based Free Tibet and US broadcaster Radio Free Asia said in separate statements late Monday.

Police in Aba, where many of the incidents have taken place, denied anyone had self-immolated. AFP calls to the local government and Kirti Monastery were not answered.

It was the fourth self-immolation reported by overseas rights groups this month as Beijing tightens security ahead of another notable anniversary -- deadly riots in March 2008 which began in the Tibet region's capital, Lhasa, before spreading to other areas.

Authorities have mounted a heavy security presence in Sichuan and other Tibetan-inhabited areas in response to the self-immolations and sometimes deadly clashes between protesters and police in recent months.

Free Tibet said last week that Chinese security forces shot three Tibetans, killing one, at a police station in Banma county in the northwestern province of Qinghai where they had gone to protest the arrest of another Tibetan.

This could not be independently confirmed and a local policeman contacted by AFP denied the shooting had taken place.

"That's a rumour. There was a criminal case, but police didn't shoot anyone. That's all I can tell you," he told AFP.

Many Tibetans in China complain of religious repression, as well as a gradual erosion of their culture, which they blame on a growing influx of majority Han Chinese in areas where they live.

China, however, denies this and says Tibetans are leading better lives than ever before thanks to huge investment in infrastructure, schools and housing.

Free Tibet said the incident took to 26 the number of people who have set themselves on fire in Tibetan-inhabited areas of China in the past year.

Beijing accuses the Dalai Lama, Tibet's spiritual leader, of inciting the self-immolations in a bid to split Tibet from the rest of the nation.