Two Tibetan teenagers died after setting themselves on fire outside a monastery in the Sichuan province in southwest China on Monday, foreign rights groups said.

The deaths of Lobsang Kalsang, an 18-year-old monk and Damchoek, a 17-year-old former monk, have raised the death toll of people who have died since 2009 due to self-immolations as a protest against the Chinese rule to 51, a London-based group Free Tibet said.

The young monks shouted anti-China slogans after setting themselves ablaze, Radio Free Asia (RFA) reported citing India-based monks who had information about the incident. Both were taken to the Barkham hospital by the Chinese authorites, where they died within hours of self-immolation.

"Witnesses saw them run about 20 steps with their bodies on fire, and then they fell to the ground," monks Kanyag Tsering and Lobsang Yeshe were quoted saying. The monks said sounds of "Ki! Ki!" a Tibetan battle cry could later be heard coming from the flames.

Damchoek was the nephew of his fellow self-immolator and was the younger brother of a nun named Tenzin Chodron who had died in an earlier protest, RFA reported.

In a separate incident, a 39-year-old Tibetan nun who staged a protest Saturday in Sichuan's Kardze prefecture was detained by the police, reports said.

China calls the self-immolators "terrorists" and reportedly released a documentary in mid-May accusing the Dalai Lama of orchestrating a wave of self-immolations by Tibetans.

However, the Tibetan government-in-exile has denied that the Dalai Lama or other exiled Tibetan monks are responsible for inciting the protests.

At least nine people have set themselves on fire in Tibetan-dominated areas in August, the rights group said.

In May, Chinese security officials detained about 600 Tibetans in Lhasa following multiple self-immolation protests against the Chinese control over Tibet.

Several rights organizations reported in late February that foreign journalists had been banned from entering Tibet as part of the Chinese efforts to prevent international news organizations from covering Tibetan demonstrations.

A crew from CNN was arrested at a toll barrier in the Sichuan province in the last week of January and was prevented from traveling to neighboring Tibet, Reporters Without Borders (RWB) reported.

Free Tibet said China was going to "great lengths to create an information blackout in Tibet, banning international journalists and observers, cutting telephone lines, blocking the internet and meting out severe punishments on Tibetans for sharing information."