Releasing a report rating countries' efforts at stopping the trade in endangered species, WWF said elephant poaching was at crisis levels in central Africa while the survival of rhinos was under grave threat in South Africa.
In parts of Asia, rhino horns are highly prized for their use in traditional medicines -- some believe they can cure cancer -- while elephants' ivory has for centuries been regarded as a precious decoration.
Global efforts to stem the trade have been under way for years, but China, Thailand and Vietnam are allowing black markets in various endangered species to flourish by failing to adequately police key areas, according to WWF.
It said Vietnam was one of the countries of most concern, giving it a worst-possible "red" score for failing to stem the trade in rhino horns as well as tiger parts.
"It is time for Vietnam to face the fact that its illegal consumption of rhino horn is driving the widespread poaching of endangered rhinos in Africa," said WWF's global species programme manager, Elizabeth McLellan.
"It must crack down on the illegal rhino horn trade."
WWF said Vietnam was the top destination for rhino horns illegally imported from South Africa.
It described South Africa as the "epicentre" in an African rhino poaching crisis, despite strong government efforts there that began in 2009 to stop the killings.
A record 448 rhinos were poached in South Africa in 2011, and this year could be even worse with 262 already lost from January to June, according to WWF.
The wildlife group accused the Vietnamese government of doing very little to stop rhino horns from being imported, describing penalties in Vietnam for buying them as not nearly strong enough to act as a deterrent.
It also said Vietnamese diplomats had been arrested or implicated in South Africa for trying to buy rhino horns.
WWF said Chinese authorities should be recognised for their strong and effective efforts to stop the rhino horn trade within their borders.
But it accused China and Thailand of being among the worst culprits in allowing the illegal trade of elephant tusks.
"Tens of thousands of African elephants are being killed by poachers each year for their tusks, and China and Thailand are top destinations for illegal African ivory," WWF said.
WWF urged China to improve its enforcement procedures and warn Chinese nationals they would face severe penalties if they were caught illegally importing ivory from Africa.
WWF said China banned using rhino horn for traditional medicines in 1993, and authorities had followed through with periodic crackdowns that were effective in stopping it being sold in pharmacies.
China has also made genuine efforts overall to stop the illegal trade of endangered species' parts, but elephants' ivory remained a big problem because of the huge demand in the world's most populous country, it said.
In Thailand, WWF said the main problem was a unique law that allowed the legal trade in ivory from domesticated elephants.
In reality, this was a "legal loophole" that allowed indistinguishable illegal African ivory to be sold openly in upscale boutiques, it said.
The conservation group said there were some bright spots around the world, with India and Nepal receiving a best-possible "green" score for their efforts to stem the trade in elephants, rhinos and tigers.
WWF said significant efforts had been made globally to save tigers following a summit in Russia two years ago that attracted leaders from the 13 countries with wild populations of the endangered animal.
Still, it warned more than 200 tiger carcasses were being detected each year on the global black market.
"With as few as 3,200 tigers remaining in the wild, every tiger poaching death is a major concern," it said.
Explore further: Rhino poaching surges in Asia, Africa