A panda research centre in China has released four 'well-selected' pregnant giant pandas into the wild, expecting them to deliver their cubs out of captivity.

The Wolong China Giant Panda Protection and Research Centre, in Sichuan Province, has returned the four giant pandas - Zi Zhu, Ying Ping, Cao Cao and Zhang Ka - at a woodland near a training base, which was built to help giant pandas live in the wild, an expert with the centre said.

This is the second phase of a plan to gradually release captive-bred giant pandas into the wild. The first phase, launched in 2003, suffered a setback when Xiang Xiang, a five-year-old male giant panda, was found dead in 2007 after he was returned to the wild.

The giant pandas, aged four to five, were expected to give birth and live in the 20,000 sq metre forest until the cubs are aged three to four.

'All of the carefully-chosen pandas have experience of living in the wild and three of them have given birth to cubs. We hope the mothers can teach their cubs life skills to help them survive in the wild,' Tang Chunxiang was quoted as saying by the Global Times.

The would-be mothers, previously kept in captivity, will produce and breed the cubs on their own, while the workers of the centre will observe them through surveillance cameras, he said.

'If they need help, the workers will show up dressed in costumes that make them look like giant pandas, in order to reduce the animals' reliance on humans,' he said.

The workers would also simulate sounds and smells of panda's natural enemies, in a bid to improve their vigilance and raise their chances of surviving in the wild, he said.

Giant pandas, known for being sexually inactive, are among the world's most endangered animals.

Around 1,600 giant pandas live in forests in Sichuan, Shaanxi and Gansu provinces and another 290 are under captive breeding.