Six captive-bred pandas will be freed into an enclosed forest in southwestern China next year in the first mass release of the highly endangered animals, the official Xinhua news agency said Wednesday.

It said the six, aged two to four, had been chosen from 108 captive-bred animals at the Chengdu Research Base of Giant Panda Breeding, and researchers believed the mass release would increase their chances of survival.

A total of 10 pandas have been individually released since 1983, it said, but only two remain in the wild, with the breeding centre taking six back after they lost weight, one found dead and the other also believed to have died.

Despite three years of preparatory training the confirmed fatality, Xiang Xiang, a five-year-old male, was found dead a year after his 2006 release, following a fight with wild pandas in a remote part of a nature reserve.

"Human-raised pandas have great difficulty surviving in the wild," Xinhua said, citing a statement from the breeding centre.

China engages in "panda diplomacy", using the endangered but iconic bears as diplomatic gifts to other countries, and also runs a lucrative trade hiring the animals out to foreign zoos.

Only around 1,600 remain in the wild in China, with some 300 others in captivity.