Wild giraffe numbers have plummeted by 40 percent in the past three decades and the species is now "vulnerable" to extinction, a top conservation body said Thursday.

The population of the world's tallest land mammal dropped to below 100,000 in 2015, mainly due to habitat loss and illegal hunting, the International Union for the Conservation of Nature (IUCN) reported.

"These majestic land animals are undergoing a silent extinction," Julian Fennessy, co-chairman of the IUCN's specialist group on giraffes, said in a statement.

Previously, giraffes had been classified as a species of "least concern" on the IUCN's Red List, which tracks the conservation status of fauna and flora.

The long-necked leaf eaters are spread out across southern and eastern Africa, with smaller pockets in west and central Africa.

Of nine distinct subspecies, three have shown an increase in population. 

But five experienced a sharp decline, with one remaining stable.

Numbers have crashed in 30 years from an estimated 157,000 to about 97,500 last year, the IUCN said.

The main culprit is the growing human population, which has caused a spike in poaching and encroachment upon the giraffe's natural habitat.

"As one of the world's most iconic animals, it is time that we stick our neck out for the giraffe before it is too late," said Fennessy.

The report was part of an larger update of the Red List, unveiled at a meeting in Cancun, Mexico of the 196-nation Convention on Biological Diversity.