"Although 15,589 species are known to be threatened with extinction, this greatly underestimates the true number as only a fraction of known species have been assessed," said Craig Hilton-Taylor, the IUCN's "Red List" program officer.

"There is still much to be discovered about key species-rich habitats, such as tropical forests, marine and freshwater systems or particular groups, such as invertebrates, plants and fungi, which make up the majority of biodiversity."

Even the depths of the world's oceans are providing little refuge to marine species, which are being over-exploited to the point of extinction. Nearly 20 percent of shark and ray species so far assessed were found to be threatened, the IUCN said.

On a positive note, scientists from the world's largest conservation organization said steps taken to preserve some "at risk" species appeared to have borne fruit.

For instance, a quarter of the world's threatened birds had benefited from conservation measures, they said.

Noteable improvements in the 2004 "Red List" include the European otter, which has moved from 'vulnerable' to 'near threatened', and the Christmas Island Imperial pigeon, once 'critically endangered' but now only 'vulnerable'.

However, overall, the list is more marked for its losers since 2003 than its winners.

Examples include the now-extinct St Helena olive, the Hawaiian crow, which has become 'extinct in the wild', and the Balearic shearwater and giant Hispaniolan galliwasp lizard, which are now both 'critically endangered'.

The 56-year-old green group said world leaders were waking up to the importance of conservation, but that more must be done.

"Governments are starting to realize the value of biodiversity and the critical roles it plays in their people's well-being," said David Brackett, head of the IUCN's Species Survival Commission.

"Species provide food, medicine, fuel and building materials. They help filter water, decompose waste, generate soil and pollinate crops. Recognition of this is growing but governments need to mobilize far more resources," he said.