BEIJING (Reuters) - The number of Tibetans who have set themselves on fire in protest at Chinese rule of their homeland has topped 50 after two teenagers burned to death in a southwestern corner of the country, a rights group said.

The pair, one an 18-year-old monk and the other a 17-year-old former monk, died on Monday after setting themselves on fire outside the Kirti monastery in Ngaba, a heavily Tibetan part of Sichuan province, the London-based Free Tibet group said.

Their protest brings to 51 the number of Tibetans who have set themselves alight since 2009, when the burnings first began, Free Tibet said in an emailed statement dated August 27.

At least half of them are believed by rights groups to have died, while scores of other Tibetans have been reported detained by security forces after protests that follow the burnings.

"Free Tibet has grave concerns for the well being of the hundreds of Tibetans who we know are in detention following protests," Free Tibet Director Stephanie Brigden said.

Calls seeking comment to the government in Ngaba, known as Aba in Chinese, were not answered.

China has branded the self-immolators "terrorists" and criminals, and has blamed the exiled Tibetan spiritual leader, the Nobel Peace Prize-winning Dalai Lama, for inciting them.

Beijing considers the Dalai Lama a separatist, while he says he merely seeks greater autonomy for his Himalayan homeland, and denies supporting violence.

Activists say China tramples on religious freedom and culture in Tibet, which has been ruled with an iron rod by the Chinese since 1950. China rejects such criticism, saying its rule ended serfdom and brought development to a backward area.

(Reporting by Ben Blanchard; Editing by Daniel Magnowski)