An 18-year-old Tibetan nun died after setting herself on fire in protest at Chinese rule in Tibet, activists said on Sunday, adding to a fast growing list of self-immolations in ethnically Tibetan areas of China.

Chinese army troops moved in after Tenzin Choedon set herself alight at the Mamae convent in China's southwestern Sichuan province, said Kanyag Tsering, a monk in Dharamsala, India, where the Tibetan government-in-exile is based.

Soldiers surrounded the nunnery and sealed it off, and nothing more is known of the situation inside, said Kanyag.

Activist group Free Tibet also reported the burning, saying in an email that the nun died after being taken away by Chinese security forces on Saturday night.

No other details were immediately available and the report could not be independently verified. A member of staff with local police said we do not have any information about this when contacted by telephone.

The nun's convent in Aba county, east of Tibet, has been at the centre of pro-Tibetan protests in recent months. She is the 22nd Tibetan in less than a year, and the sixth this week, to set themselves on fire, Free Tibet director Stephanie Brigden said.

We know many more Tibetans are willing to give their lives and Tibetans are protesting in the streets, she said.

The self-immolations are a small but potentially destabilising challenge to China's regional policies, and the government has branded those who set themselves alight as terrorists.

Activists say China violently stamps out Tibetan religious freedom and culture in Tibet, the mountainous region of western China which has been under Chinese control since 1950.

Protests by self-immolation have become more common in Tibet and in restive, ethnically Tibetan regions of China and at least 14 Tibetans are believed to have died from their injuries. Exiled Tibetan leaders say they fear a crackdown in the region to coincide with the Tibetan new year on February 22.

We have reports that hundreds of convoys carrying Chinese military personnel with automatic machineguns are moving towards Tibet, Lobsang Sangay, prime minister of the government-in-exile, said last week.

(Reporting by Terril Yue Jones, Abhishek Madhukar and Sally Huang; Editing by Ed Lane and Ben Harding)