African elephants face the worst crisis since global trade in ivory was banned almost a quarter-century ago, with the risk of extinction rising in worst-hit nations, conservationists said Wednesday.

Illicit trade in ivory -- driven by organised crime rings -- has doubled since 2007 and more than tripled over the past 15 years, experts warned on the sidelines of a major conference on endangered species in Bangkok.

"Current population estimates suggest alarming declines in elephant numbers in parts of Central and West Africa, as well as an increasing risk of the local extinction of some populations," according to the report by the UN Environment Programme and other wildlife groups.

"Previously secure populations in Eastern and Southern Africa are under growing threat, as a wave of poaching seems to be spreading east and southwards across the African continent."

The plight of Africa's elephants and rhinos is top of the agenda at a meeting of 178 member nations of the Convention on International Trade in Endangered Species (CITES) in the Thai capital this week.

Conservationists fear that 2012 was an even deadlier year for African elephants than 2011, when an estimated 25,000 of the animals perished.

Only about 420,000 to 650,000 elephants are thought to remain in Africa, the report said.

"This is the work of organised crime. You don't invest in hundreds and hundreds of tusks as a frivolous pastime," said Tom Milliken of wildlife trade protection group Traffic which co-produced the report.

"Unless CITES really scales up and takes this issue seriously, we are not going to win this thing," he said, calling for wildlife trade sanctions against countries which fail to tackle the problem.

Three African nations -- Uganda, Tanzania and Kenya -- have been identified as not doing enough to tackle the illegal trade, along with transit countries Malaysia, Vietnam and the Philippines and top markets China and Thailand, said CITES coordinator Tom De Meulenaer.

They have been asked to present credible action plans on March 14 to the convention's permanent committee, which has the power to impose the trade sanctions against offender countries for the 35,000 species under CITES protection.

Such a step is "not unlikely", De Meulenaer said.

"Can you imagine what it would mean for countries like Vietnam, or China, or Thailand for that matter, if all wildlife trade -- its orchid trade, its massive crocodile product trade -- would be stopped?" he added.

Speaking at the opening of the meeting on Sunday, Thai Prime Minister Yingluck Shinawatra pledged to work towards "an end to the ivory trade" in the kingdom but provided few details such as a timetable for such a move.