The Western Black Rhino of Africa was declared officially extinct Thursday by a leading conservation group.

The International Union for Conservation of Nature said that two other subspecies of rhinoceros were close to meeting the same fate.

The Northern White Rhino of central Africa is now "possibly extinct" in the wild and the Javan Rhino "probably extinct" in Vietnam, after poachers killed the last animal there in 2010.

A small but declining population survives on the Indonesian island of Java.

IUCN said Thursday that a quarter of all mammals are at risk of extinction, according to its updated Red List of endangered species.

'Stewards of the Earth'

But the group added that species such as the Southern White Rhino and the Przewalski's Horse have been brought back from the brink with successful conservation programs.

"Human beings are stewards of the Earth and we are responsible for protecting the species that share our environment," said Simon Stuart, chair of the IUCN Species Survival Commission.

"In the case of both the Western Black Rhino and the Northern White Rhino, the situation could have had very different results if the suggested conservation measures had been implemented," he added. "These measures must be strengthened now, specifically managing habitats in order to improve breeding performance, preventing other rhinos from fading into extinction."

Rhinos get upside-down helicopter ride to safety

The WWF environmental campaign group last month said that the Javan Rhino found dead in Vietnam in 2010 was the country's last, rendering the species all but the extinct.

Genetic analysis of 22 dung samples collected in Vietnam's Cat Tien National Park from 2009 to 2010 affirmed that the animal, found dead with a bullet in its leg and its horn removed in April 2010, was the final wild rhino in Vietnam.

Rhinoceros horns are a coveted ingredient in traditional Eastern medicine and rumored to cure or fend off cancer, although scientists say there is no evidence to support the claim.

WWF said the Javan Rhino was believed to be extinct from mainland Asia until 1988 when one was hunted from the Cat Tien area, leading to the discovery of a small population.

The IUCN did note that some species have been brought back from the brink with successful conservation programs.

Video: Hidden camera catches rare rhinos (on this page)

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.