Two teenagers burned to death in southwest China, taking to over 50 the number of Tibetans who have set themselves alight in protest against Beijing's rule, rights groups said on Tuesday.

Lobsang Kalsang, 18, a Buddhist monk, and former monk Damchoek, 17, died in hospital on Monday after setting themselves on fire in Aba town, which has become a flashpoint for such protests by ethnic Tibetans.

China's Tibetan-inhabited areas have seen an explosion in the violent form of protest since March 2011, when the self-immolation of a monk named Phuntsog at Aba's revered Kirti monastery sparked riots and a police crackdown.

The first recorded similar incident was in February 2009, and there have now been 51 such fiery protests, according to tallies compiled by overseas-based pressure groups Free Tibet and the International Campaign for Tibet.

In 2009, a young Kirti monk doused himself in oil and set himself on fire carrying an image of the Dalai Lama, Tibet's spiritual leader, before being shot by police and taken to a local hospital.

The next incident was not recorded until 2011, but since then dozens of ethnic Tibetans, most of them young monks and nuns, have set fire to themselves. Many, though not all, have died.

Experts say suicide is a major taboo in Tibetan Buddhist culture, and the immolations are a sign of growing desperation among those living in the vast and remote Tibetan plateau.

The two men in the latest incident shouted slogans condemning Chinese policies in Tibet as they set themselves alight, Radio Free Asia said, citing two India-based monks with contacts in Aba, Sichuan province.

They protested close to the Kirti monastery, which has been under intense security since Phuntsog's immolation.

An AFP reporter who managed to gain access in March saw hundreds of armed soldiers, some of them carrying fire extinguishers, lining roads, patrolling the town's narrow main street and manning road blocks.

"Tibetans' fundamental human rights are being ignored by international leaders who are afraid of risking their relationships with China," said Free Tibet director Stephanie Brigden in a statement.

"The time has come for each one of us to speak up and demand Tibetan freedom."

No one at the Kirti monastery or the hospital where the monks reportedly died could be reached by telephone, while authorities in Aba declined to comment on the latest incident.

Free Tibet said Damchoek, who like many Tibetans used only one name, was the brother of Tenzin Choedon, a teenage nun who set fire to herself in February this year.

Tibetans have long chafed under China's rule over the Tibetan plateau, saying that Beijing has curbed religious freedoms and their culture is being eroded by an influx of Han Chinese, the country's main ethnic group.

Many say that the situation has worsened dramatically since 2008 riots in the Tibetan capital Lhasa, and a recent US State Department report signalled "a marked deterioration" in Beijing's respect for and protection of religious freedom last year.

"In the Tibetan Autonomous Region and other Tibetan areas, this included increased restrictions on religious practice, especially in Tibetan Buddhist monasteries and nunneries," it said.

Beijing, however, says that Tibetans enjoy religious freedom and have benefited from improved living standards brought on by China's economic expansion.

China has accused the Dalai Lama -- who fled Tibet following a failed uprising against Chinese rule in 1959 and is vilified as a "separatist" by communist authorities -- of encouraging the protests.

The Dalai Lama has condemned self-immolations, which many Buddhists believe are contrary to their faith, and blamed them on the hardline Chinese rule of Tibetan-populated areas.