By Richard Black
Environment correspondent, BBC News website, The Hague
A rise in poaching has put some rhino populations at risk of extinction.
Across Africa as a whole, rhinos have been on the increase
The wildlife trade organisation Traffic has documented a five-fold increase in the volume of rhino horn entering the illegal market between 2000 and 2005.
The populations most affected are in western and central Africa and Nepal, with one sub-species in Cameroon believed extinct already.
However, overall, rhinos are doing well with Africa-wide numbers increasing by about 6% every year.
The Traffic report was released at the Convention on International Trade in Endangered Species (CITES) summit in The Hague.
"We are seeing an increase in the quantity of horn which is leaving the continent," said Simon Milledge, Traffic's deputy director for eastern and southern Africa.
"The main market remains in east and southeast Asia, as well as in the Middle East. It's a concern."
Poached to extinction
In the middle of the 1800s, there were probably more than a million black and white rhinos on the plains of Africa.
Rapacious hunting by European settlers brought numbers down spectacularly, and at one point the southern white was thought extinct.
Protective measures brought a reversal for both species, and in southern and eastern Africa, the revival continues, with countries such as Namibia and South Africa having found a new use for their rhinos as a tourist attraction.
Across the continent, there are now more than 14,000 white and nearly 4,000 black rhinos. Live animals can legally change hands for between $20,000 and $50,000 (£10-25,000), far more money than an illegally traded horn can bring.
The Traffic report names Cameroon, Democratic Republic of Congo and Zimbabwe as countries where protective measures have broken down.
An expedition in Cameroon last year found that the one remaining tiny population of the northern black rhino sub-species Diceros bicornis longipes had probably been poached to extinction.
DRC is home to the last four northern white rhinos (Ceratotherium simum cottoni) in existence.
Asian rhinos, meanwhile, show a mixed picture.
Indian populations are rising; but in Nepal, recent conflict has brought heavy poaching. And there are other problems in Indonesia.
"The Sumatran and Javan rhinos are very vulnerable," noted Simon Milledge.
"The greatest threat is habitat loss and the fragmentation of habitat; the threat of the horn trade is there, but it's mainly habitat issues for those two species."
CITES voted through a resolution aimed at enhancing rhino protection through greater monitoring of both the animals and the horn trade, better co-operation between African range states, and an assessment of horn stockpiles.
A Kenyan amendment that stockpiles should be destroyed was defeated.
Earlier, another Kenyan proposal, to stop the annual export of five black rhino hunting trophies by Namibia and a further five by South Africa, was defeated. The exports had been approved at a previous CITES meeting, and South Africa says that its quota brings in nearly $1m per year which can be spent on conservation.
Traffic is a joint programme of the conservation group WWF and the IUCN, which is famous for drawing up the Red Lists that document the status of the planet's flora and fauna.