China will begin sending pandas bred in captivity into a controlled wilderness area in southwestern Sichuan province next month, the most ambitious attempt to rebuild the country's depleted population of giant pandas in a natural habitat.

The first six pandas selected from 108 raised by the Chengdu Giant Panda Rehabilitation Project -- the world's largest captive bred population of giant pandas -- will be released to a protected natural area covering more than 2,000 acres (780 ha) in Dujiangyan city.

"Rather than keeping them in their enclosures, we will spend the next 50 years helping them return to their natural habitat, which is the ultimate goal of the Chengdu Panda Base," Zhang Zhihe, director of the base, told a briefing on  Wednesday.

The pandas, bred through artificial insemination, will be released in batches and monitored as they acclimatise. Those who perform well in an initial area will be released into the primary controlled wilderness area.

The first six pandas range in age from two to four and were chosen on the basis of gender ratio and health. Zhang acknowledged the effort carries risks and noted that other rehabilitation programmes have had little success.

"We have collected some experience of rehabilitation from other organisations who did that several times before. As you all know, it's a long-term procedure for transitional training of the giant panda," Zhang said.

"As we are still at the explorer stage, nobody could guarantee the successful result of this release."

In 2004, a census by the Worldwide Fund for Nature counted 1,600 pandas in the wild, most in Sichuan province.

Pandas are difficult to breed because females ovulate only once a year and can only become pregnant during that two- or three-day period.