SURPRISINGLY large populations of two  globally threatened species of monkeys have been found in Mondulkiri's Seima Biodiversity Conservation Area, the US-based Wildlife Conservation Society (WCS) said.

With an estimated 42,000 black-shanked douc langurs and 2,500 yellow-cheeked crested gibbons, all evidence suggests that Cambodia has the largest populations of the species in the  world, said Edward Pollard, a WCS scientist, in a statement released Friday.

Before this discovery, the largest-known populations of the primates were believed to be in Vietnam, where the numbers of black-shanked douc langurs and yellow-cheeked crested gibbons hovers at around 600 and 200 respectively, said WCS.

While the total population of the two primates remains unknown, their numbers started to recover in 2002 following a joint program between WCS and the Cambodian government and have been stable since 2005.

WSC contributes the large primate numbers to  successful management of the area, a cessation of logging and a gun confiscation programme. However, WSC researchers  remain concerned that other looming threats, including agro-industrial plantations and commercial mining, will jeopardise recent successes.

The WCS's census was first presented at the 22nd International Primatological Society Congress, where another survey identified Cambodia as having the worst record for declining primate numbers, with 90 percent of species struggling to survive.

The survey covered an area of 789 square kilometres.

ADDITIONAL REPORTING BY AFP