The western black rhino, a species related to the eastern black rhino pictured above, has been officially declared extinct. Richard Elmslie/IUCN

The western black rhino of Africa has been declared officially extinct, and two other subspecies of rhinoceros are close to meeting the same fate, a leading conservation group said Thursday.

The International Union for Conservation of Nature said a recent reassessments of the western black rhino had led it to declare the species extinct, adding that the northern white rhino of central Africa is now "possibly extinct" in the wild and the Javan rhino is "probably extinct" in Vietnam, after poachers killed the last animal there in 2010.

A small but declining population of the Javan rhino survives on the Indonesian island of Java, it added.

"A lack of political support and willpower for conservation efforts in many rhino habitats, international organized crime groups targeting rhinos and increasing illegal demand for rhino horns and commercial poaching are the main threats faced by rhinos," the group said in a statement accompanying the latest update of its so-called Red List of endangered species.

About a quarter of all mammals are at risk of extinction, IUCN said, adding that some species have been brought back from the brink with successful conservation programs.

The southern white rhino numbered just 100 animals at the end of the 19th century, but has since flourished and now has a population of over 20,000.

The Przewalski's horse, a type of wild horse from Central Asia, has come back from extinction after a successful breeding program in captivity.

The Red List now contains almost 62,000 species of plants and animals, whose status is constantly monitored by conservationists.