Global conservationists and policymakers meet in South Africa from Saturday to chart a way forward in the fight against escalating wildlife trafficking that could drive some species to extinction.

The plight of Africa's rhino and elephants, targeted for their horns and tusks, is expected to dominate 12 days of talks in Johannesburg on the Convention on International Trade in Endangered Species (CITES).

Illegal wildlife trade is valued at around $20 billion a year, according to CITES, and is ranked the fourth largest illicit business in the world after arms, counterfeit goods and human trafficking.

The gathering is expected to assess whether to toughen or loosen trade restrictions on some 500 species of animals and plants.

"Much of the international attention will focus on the ivory of the African elephant and the horn of the Southern White Rhinoceros, and on combating their illegal trade," said CITES secretary general John Scanlon ahead of the event.

CITES banned the trade in rhino horn 40 years ago, but prohibition has not reduced illicit hunting, which has recently boomed in South Africa.

Around 5,000 white rhino -- a quarter of the population -- have been slaughtered over the past eight years, with the majority killed in South Africa, home to 80 percent of the world's rhino.

Rhino poaching is driven by insatiable demand in Vietnam and China for the horn, which is mistakenly believed to have medicinal powers curing everything from hangovers to cancer.

- Divisive proposal -

Swaziland has put forward a divisive proposal asking for the total ban on international trade in rhino horn to be eased so that it can sell its stockpile of legally and non-lethally harvested horns.

The plan is backed by some private rhino owners who say the black market can be ended by supplying legal horn sawn off anaesthetised live animals and the funds used to fund conservation.

"It's costing an absolute fortune. Just every cent we have goes into protecting the rhino, it's not sustainable for the long term," South African private rhino owner Lynne MacTavish, told AFP at Pilanesburg game reserve where an anti-poaching helicopter was unveiled this week by non-profit organisation Rhino911.

But Swaziland's proposal stands little chance of success in the face of a fierce campaign by conservation groups who fear legalised trade would only boost the appetite for horn.

The Humane Society International has urged Swaziland to withdraw its proposal, citing the strictly controlled legal ivory trade that has opened up opportunities for the laundering of illicitly acquired tusks.

"As a consequence, African elephants have undergone a catastrophic decline," it said in the letter to Swazi King Mswati III, warning that "the same will happen to rhinos if the legal international horn trade is approved."

- Ivory trade -

Results from the world's largest wildlife census, released last month, showed that savannah elephant populations have declined by 30 percent over seven years to 2014.

CITES in 2008 allowed for a one-off ivory sale, a move which some anti-trade advocates blame for the spike in elephant poaching.

Zimbabwe and Namibia are asking that the ban on commercial ivory trade be lifted to allow them to sell stockpiles accruing from elephants dying naturally and those confiscated from poachers.

Proceeds would go towards local villagers living near elephants who are helping to protect the animals.

"We are supporting Namibia and Zimbabwe on their proposal because we do think it is very understandable," said South Africa's Environmental Affairs Minister Edna Molewa.

But there are fears that allowing a resumption in trade -- albeit just for the two countries -- would signal that trade is opening up and fuel demand.

Vietnam, a key consumer of rhino horn, is expected to come under severe scrutiny at the CITES conference, which is held every three years.

"Vietnam's poor law enforcement record speaks for itself: ending the illegal rhino horn trade and helping to save Africa's rhinos is clearly not a priority for the government," said Ginette Hemley, WWF head of delegation.

"CITES must take a tough line," by forcing Vietnam to swiftly prosecute traffickers and traders or face trade sanctions in all other regulated species, she said.

Rosewood plundered from forests, lions, pangolins and sharks will also be hotly debated on an agenda that Molewa admitted was full of "controversial and thought-provoking" dilemmas.

Organisers hope one strong message to emerge will be that corruption must be tackled if trafficking is to be reduced.

The meeting, described by organisers as the largest and most important CITES event since its founding in 1975, will be attended by around 3,500 delegates from 182 member nations.