A Tibetan mother-of-four has died after setting herself on fire in south-west China, activists and exiles say. The UK-based Free Tibet group said the woman set herself on fire near the Kirti Monastery in Aba prefecture in Sichuan province. It is the latest in a spate of self-immolations by ethnic Tibetans in apparent protest against Chinese rule. Security in Tibetan areas of China has been further tightened ahead of sensitive anniversaries. The Kirti Monastery has been at the forefront of pro-Tibetan protests in recent months. Reports said that the woman, with her body in flames, shouted for Tibetan freedom and the return of spiritual leader the Dalai Lama before she died. Local officials have not commented on the incident. More than 20 Tibetans have burned themselves to death in the past year, despairing at what they say is repressive Chinese rule and the erosion of their culture. China has poured money into Tibetan-inhabited areas, seeking to win them over by boosting the economy. But it has also flooded the same areas with police, increased surveillance at monasteries and partially blocked the internet and mobile phones. Foreign journalists caught trying to reach the scene of much of the unrest - in the west of Sichuan province - have been turned back or detained. March is a month of sensitive Tibetan anniversaries including that of the Dalai Lama's flight into exile in 1959. BBC Asia-Pacific editor Viv Marsh says Chinese authorities will be keen to avoid protests during a parliamentary meeting in Beijing this week ahead of a big leadership transition later this year.