(CNN) -- Researchers have discovered a previously unknown group of rare monkeys in the forests of Vietnam.

The Tonkin snub-nosed monkeys were so skittish, researchers captured a photo of only one: an adult male.

Several biologists caught fleeting glimpses of about 15 or 20 Tonkin snub-nosed monkeys in a remote area near the Chinese frontier, the wildlife conservation group Fauna & Flora International said Thursday.

The "bizarre-looking" monkeys -- on the brink of extinction -- were so skittish around people that researchers were able to snap a photo of just one of them: an adult male scampering through the trees.

The monkeys were "very sensitive to the presence of people, giving warning signs to one another and fleeing" whenever biologists approached, the group said in a statement.

"It was apparent that the monkeys associated humans with danger -- perhaps due to ongoing threats from hunters," the group said.

So few Tonkin snub-nosed monkeys have survived in the wild that scientists thought until recently that they were extinct. Now they estimate that roughly 200 remain, mainly in parts of northern Vietnam near the Chinese border.

Hunters with a taste for bush meat and the loss of habitat have pushed the Tonkin snub-nosed monkey toward extinction, according to the International Union for the Conservation of Nature.

It classifies the primate as critically endangered "because its population size is estimated to number fewer than 250 mature individuals, with no subpopulation greater than 50 mature individuals, and it is experiencing a continuing decline."

Fauna & Flora said it is working with a variety of groups to improve the livelihoods and "reduce human pressures on the forest ecosystem" in an effort to safeguard the newly discovered group, which was spotted in a patch of forest in the Quan Ba district of Vietnam's Ha Giang province.

The sighting thrilled conservation biologist Le Khac Quyet, described by Fauna & Flora as "one of the few people in the world who can claim to be an expert on this mysterious species" and as the person credited with discovering the new group of that species.

"When I saw the Tonkin snub-nosed monkeys ... I was overjoyed," he said in the Fauna & Flora statement.

"There is still time to save this unique species, but with just 200 or so left and threats still strong, we need to act now."

All About Vietnam • Endangered Species