By Alex Kirby
BBC News Online environment correspondent
The clock now stands at one minute to midnight for the world's four great ape species, the United Nations says.
Satellites are pinpointing gorilla forests
It is launching an appeal for $25m, the minimum it says is needed to avert their extinction within a few decades.
All the great apes - gorillas, chimpanzees, bonobos (pygmy chimps), and orang-utans - face a very high risk of extinction within 50 years at most, the UN says.
It hopes to establish areas where ape populations can stabilise or even grow, if it manages to raise enough money.
The appeal is being launched at a meeting in the French capital, Paris, on 26 November. It has been called by Unesco (the UN Educational, Scientific and Cultural Organisation) and Unep, the UN Environment Programme.
The two agencies, the co-ordinators of Grasp, the Great Apes Survival Project, have invited delegates from the 23 African and south-east Asian states where the apes live, and from possible donor countries.
Dr Klaus Toepfer, Unep's executive director, said: "$25 million is the bare minimum we need, the equivalent of providing a dying man with bread and water.
Time is short for all the apes
"The great apes share more than 96% of their DNA with humans. If we lose any great ape species we will be destroying a bridge to our own origins, and with it part of our own humanity."
Unesco's director-general, Koichiro Matsuura, said: "Great apes form a unique bridge to the natural world. The forests they inhabit are a vital resource for humans everywhere.
"Saving the great apes and the ecosystems they inhabit is not just a conservation issue, but a key action in the fight against poverty."
Contending with humans
Unesco says research suggests the western chimpanzee has already disappeared from Benin, Togo and Gambia, and could soon vanish from another west African country, Senegal.
There are only about 2-400 chimpanzees left there, slightly more in Ghana, and fewer than 200 in Guinea-Bissau.
The orang-utans' plight is acute
The main threats to all the apes are of human origin: war, poaching, and the live animal trade.
Human encroachment into the forests is increasing, and outright forest destruction leaves them no hiding place.
If road-building, construction of mining camps and other development continues at present levels, Unep says, less than 10% of the remaining forest where the African apes live will be left relatively undisturbed by 2030.
Checking forest loss
By then, it says, there will be almost no habitat left intact in south-east Asia, raising acute fears for the orang-utan's survival.
Because many great apes live in remote and inaccessible areas, Unesco is working with the European Space Agency to use satellites or remote sensing to monitor how fast ape habitat is disappearing.
Chimps are in retreat in west Africa
The project is initially mapping the habitat of the mountain gorilla. Only about 600 survive in Uganda, Rwanda and the Democratic Republic of Congo (DRC).
It will also compare satellite image archives to assess changes in gorilla habitat since 1992 in the Virunga National Park (DRC) and Bwindi Impenetrable National Park (Uganda).
Unesco is working with local rangers on improving law enforcement and monitoring in all five of the DRC's world heritage sites, which are home to several great ape species.
But Samy Mankoto of Unesco said: "Law enforcement is an essential but single element in any conservation effort.
"We cannot just put up fences to try and separate the apes from people. The apes play a key role in maintaining the health and diversity of tropical forests, which people depend upon.
"They disperse seeds throughout the forests, for example, and create light gaps in the forest canopy which allow seedlings to grow and replenish the ecosystem."