Researchers at a giant panda breeding base in southwest China's Sichuan Province released two pregnant giant pandas into a semi-wild environment near the base on Thursday, as part of the base's efforts to help more captive-bred pandas adapt to the wild.

The pandas, nicknamed "Ying Ying" and "Su Lin," were the second and third captive-bred pregnant pandas to be released into the semi-wild training ground near the base, said a spokesman with the Wolong Giant Panda Protection and Research Center.

The prospective mothers were moved to two separate semi-wild training grounds on the Wolong Nature Reserve, said the spokesman.

In July 2010, a pregnant panda nicknamed "Cao Cao" was released into a semi-wild training ground on the reserve during her pregnancy. A month later, she gave birth to "Tao Tao," a male cub.

Cao Cao and Tao Tao were found to be healthy and had acquired the basic skills they would need to survive in the wild, said the spokesman.

In February, Cao Cao and her cub were transferred to a larger training base.

After the success of Cao Cao, researchers planned to expand their training program by releasing six more pregnant pandas, including Ying Ying and Su Lin, into a semi-wild environment in 2011.

Two more semi-wild training grounds have been built to accommodate the pandas. Zoologists will keep a close eye on the pandas by using infrared surveillance cameras installed on the grounds.

China's plan to save its endangered pands by releasing captive-bred pandas back into the wild began in 2003, with a male cub nicknamed "Xiang Xiang."

Xiang Xiang was released into the wild in 2006, but was found dead 10 months later in a remote corner of the Wolong Nature Reserve. He had apparently been attacked and killed by wild pandas native to the area.

The panda training program was resumed last year at two panda research centers in Wolong and Chengdu, capital city of Sichuan Province.

Giant pandas have faced reproductive struggles in captivity. Only about 24 percent of female captive pandas end up giving birth, posing a serious threat to repopulation.