Decades of conservation efforts to save rhinos are coming undone, as surging demand for their horns in Asian traditional medicine has spawned a vast criminal trade powered by poaching.

South Africa is the epicentre of the poaching battle. A conservation success story, the country is home to 70 to 80 percent of the world's rhinos.

In 2007, 13 rhinos were poached. Last year the number hit 448, and more than 200 have already been killed this year.

In Kenya, Zimbabwe and other countries, poaching is also on the rise, but at a less dramatic pace.

The southern Africa Rhino Management Group warns that if current trends continue, the number of deaths will outstrip births, sending the rhino population on a downward spiral.

The massive herbivores that seem to have stepped right out of pre-historic times were nearly killed off during the 19th and 20th centuries.

Through conservation efforts, and lucrative private game farms, white and black rhino populations have rebounded. Africa now has an estimated 20,700 white rhinos, and 4,800 black.

Poaching is now threatening that fragile success. Demand for rhino horn in Asian traditional medicine is booming. On the black market, the horns are literally worth their weight in gold: about 50,000 euros ($66,000) per kilo.

China, once a major buyer, has taken steps to observe the international ban on trade in rhino parts, according to TRAFFIC, which monitors illicit wildlife trade.

"Right now it's very rare in China to use rhino horn in medicine," spokeswoman Luo Anan told AFP. "People's attitudes have shifted since 1993, when the use of rhino horn in Chinese medicine was banned, and they now use other things, such as buffalo horns, instead."

-- 'Demand mainly from Vietnam' --

"There is relatively little demand in China now, especially for medicinal use. The demand mainly comes from Vietnam."

In Vietnam, the horn is ground into a powder that is believed to treat fevers, stay youthful and even cure cancer. None of those uses have any scientific basis. Rhino horns are mostly made of keratin, the same material in human fingernails.

Driven by the huge profits, poachers organised into criminal networks that infiltrated even into conservation efforts. In February, four rangers at the famous Kruger National Park were arrested on poaching charges.

All it takes is a cell phone and a GPS to track rhinos.

Once spotted, some gangs operate by helicopter, others by foot. The rhino is darted, or simply gunned down. A few minutes later, the horn is hacked off.

Animals that survive the initial attack usually die of their injuries. When mothers are killed, their young usually dies soon afterward from lack of care.

South Africa last year deployed soldiers along the borders, even inside Kruger, in a bid to stop the slaughter.

Guides who use radios to alert each other to lions, leopards and elephants are no longer allowed to signal the location of rhinos.

Some private reserves that can't afford armed patrols have started de-horning rhinos. That's a difficult procedure in itself, and offers no long-term protection: the horns grow back.

Others are injecting horns with poisons or colourants.

Revived debate on legalising trade in rhino horns has so far failed to convince experts or conservationists.

What everyone agrees is that the only long-term solution lies in reducing demand, with greater law enforcement in importing nations and better education for consumers.