Conservationists believe about 2,000 rare apes are living out of sight in a remote lowland region of East Kalimantan.

The find, if confirmed, will raise the number of known orang-utans in the world by about 10%.

The discovery of a large, biologically viable, previously unsurveyed orang-utan population in East Kalimantan is very significant

Dr Birute Galdikas, Orang-utan Foundation International

It offers hope of saving the endangered primate from extinction in the wild.

The claim, by researchers from the US-based conservation group Nature Conservancy, is based on signs of nests made by the apes.

A survey of forest terrain suggests between 1,000 and 2,500 orang-utans are living in the area unnoticed, until now.

"This group could be one of the three largest populations in the world," said Linda Engstrom on the group's website.

Engstrom, and colleague Bhayu Pamungk, spent four months searching dense areas of forest with teams of local villagers they had trained.

Logging threat

Orang-utans, like gorillas and chimpanzees, are often described as the closest relatives of humans.

They are in grave danger of extinction, because their habitat is under threat from illegal logging, forest fires and gold mining.

Many apes were orphaned in the forest fires of 1997 and 1998

Wild orang-utans exist only on two south-east Asian islands, Borneo and Sumatra.

An estimated 9,000 orang-utans survive in northern Sumatra, mainly near one national park, while some 10,000 to 15,000 orang-utans remain in Borneo.

Primate experts have predicted that the apes will be found only in zoos by the year 2020 unless immediate steps are taken to protect them.

According to a report by the United Nations Environment Programme (Unep), the annual 5% loss of habitat means there will be virtually no intact forest left for them by 2030.

The orang-utan

The second largest of the great apes Unlike other apes, they tend to live alone, in pairs or in small family groups Orang-utans spend their lives among the trees and are the largest tree-living mammals

London-based Orang-utan Foundation International says there has been a dramatic decline in wild orang-utan populations throughout the tropical rainforests of Borneo and Sumatra.

"The discovery of a large, biologically viable, previously unsurveyed orang-utan population in East Kalimantan is very significant," said scientist and conservationist Dr Birute Galdikas, the president of the foundation.

"This find extends the orang-utan's known range and gives us hope that we can save orang-utan populations from extinction in the wild."