Two Tibetan teenagers have died after setting themselves on fire outside a monastery in the latest protest against Chinese rule, according to an overseas rights group.

The deaths pushed to 51 the number of confirmed self-immolations since 2009, London-based Free Tibet said in a statement.

The group said 18-year-old monk Lobsang Kalsang and 17-year-old former monk Damchoek set themselves on fire on Monday outside Kirti monastery in Aba county in the south-west Chinese province of Sichuan. They died later that day in hospital, Free Tibet said.

Citing witnesses, Free Radio Asia said the two teenagers shouted slogans condemning Chinese policies in Tibet.

Police in Aba county said on Tuesday they had no information on the self-immolations, which are rarely reported by Chinese state media. There have been at least 27 self-immolations in Aba, according to an earlier tally by the International Campaign for Tibet.

Supporters say the self-immolations are done in protest against Beijing's heavy-handed rule in Tibetan regions and to call for the return of the Dalai Lama. China has blamed the Tibetans' exiled spiritual leader for inciting the deaths, but the Dalai Lama denies the claim. He has never publicly supported or denounced the acts.

Lobsang Sangay, the political leader of Tibetan exiles, said earlier this month the incidents were against the movement's commitment to non-violence but it was his duty to highlight why the protesters were dying.

He expressed his disappointment that the self-immolations by Tibetans have not received the same international attention as the similar suicide of a Tunisian man that sparked the Arab spring.

In Washington, the state department spokeswoman Victoria Nuland said on Monday the US had called on Beijing to "meet the legitimate grievances of the Tibetan people within China to protect their human rights, to protect their way of life, and to work on these issues through dialogue".