Almost nine-tenths of Vietnam's most important sites for bird conservation are at risk from hunting and trapping, experts say.
By Alex Kirby
BBC News Online environment correspondent
The warning comes from BirdLife International, a global alliance of ornithologists working in more than 100 countries.
Black-faced spoonbill: Under local protection (Image: Martin Hale)
It says the danger comes mainly from a huge rise in the trade in wildlife, and from agricultural pressure.
But BirdLife praises the Vietnamese Government's work to establish protected areas for wildlife.
With the Institute of Ecology and Biological Resources, Hanoi, BirdLife has published a guide, Key Sites For Conservation In Vietnam.
It says 56 of 63 of the most critically important places for bird and biodiversity conservation (88%) are affected by illegal hunting and trapping.
Second to that as a threat is more intensive farming, which affects 43 of the sites.
The guide says this is especially serious for wetlands, including coastal mudflats and wet grasslands in the Mekong delta.
Richard Grimmett, head of BirdLife's Asia division, said: "Wild animals such as bears, primates and turtles are being trapped or shot in large numbers.
Golden-winged laughingthrush (Image: BirdLife)
"This is largely because of the massive increase in wildlife trade following the opening up of Vietnam's borders and road development into remote areas.
"However, it is not all gloomy news thanks to the far-sighted commitment of the Vietnamese Government to biodiversity conservation, which has led to the recent establishment of more than six protected areas to protect Vietnam's unique wildlife."
BirdLife has also welcomed the recent establishment of site-support groups, which bring local people together to plan the long-term sustainability of the area, for themselves and for wildlife.
The study on which the guide is based was funded by the Danish development agency Danida.
The Danish ambassador to Vietnam, Bjarne Sorensen, said: "In recent years, Vietnam has captivated the world through the discovery of a remarkable series of new bird and mammal species.
"Sadly, however, as in many other regions of the world, rapid population growth and economic development are threatening the rich biodiversity for which Vietnam is so rightly famed."
The illegal wildlife trade is a huge threat (Image: J Eames/BirdLife)
Earlier fieldwork by BirdLife to identify Vietnam's key conservation sites led to the discovery of three species of bird new to science - the black-crowned barwing, and the golden-winged and chestnut-eared laughingthrushes.
The country is also home to an extremely small population of highly endangered rhinos.
New mammal species found in recent years in south-east Asia include the world's smallest deer, in Burma, and a striped rabbit in the mountains straddling the border between Laos and Vietnam.
In the last few years, a forest pig and a hoofed animal like an antelope, the saola, have also been found in the region.