A teenager has become the third Tibetan to self-immolate in China in as many days, exile groups said, as Beijing tightens security ahead of the sensitive anniversary of deadly 2008 riots.

The 18-year-old man shouted anti-government slogans as he set himself alight near a government office in southwestern Sichuan province's Aba prefecture, Free Tibet and International Campaign for Tibet said in separate statements late Monday.

The man, whose name was given as Dorjee, died at the scene and his body was removed by security personnel, London-based Free Tibet said.

Police in Aba, which has a large population of ethnic Tibetans and has become a flashpoint for anger against perceived repression of Tibetan culture, language and religion, declined to comment when contacted by AFP.

The protest came a day after a mother of four died after setting herself on fire on Sunday in Aba, according to reports by the same groups.

Another teenager -- a girl reported to be aged between 16 and 19 -- set herself alight on Saturday in the northwestern province of Gansu, which borders Sichuan and also has a large population of ethnic Tibetans.

Overseas rights groups say the escalation in self-immolations -- a relatively new phenomenon -- illustrates the growing desperation of ethnic Tibetans, who have long bristled at China's rule over the vast Himalayan region.

Rights groups say more than 20 people have set themselves on fire in Tibetan-inhabited areas of China over the past year, mostly in Sichuan, in protest at what they see as repressive Chinese rule.

Beijing has substantially increased security in its Tibetan-inhabited areas since the deadly riots which began in the Tibet region's capital, Lhasa, in March 2008 before spreading to other areas.

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

But tensions have increased markedly in recent months, and western parts of Sichuan -- which borders the Tibet region and has a large population of ethnic Tibetans -- have been hit by deadly bouts of unrest.

Authorities have responded by increasing their surveillance of monasteries and cutting some phone and Internet communications.