Two plantation workers arrested in Indonesia for shooting a critically-endangered orangutan multiple times before beheading the animal and throwing its body into a river, have told police they acted in self-defence.

The men admitted to killing the animal, whose body was discovered in January. Photos of it quickly began to circulate online, sparking outrage from animal rights group and decent human beings.

Central Borneo police chief Anang Revandoko told AFP (via Channel News Asia): "They claimed they killed the orangutan because they were scared to see such a big animal suddenly coming their way."

The two men, aged 32 and 41, face up to five years in prison under Indonesia's conservation laws.

Revandoko said an investigation into the killing was still ongoing after the machete used to main the animal was discovered.

"The men shot the animal multiple times but [they claim] it didn't die," he added. "One of them then decapitated it from behind. Then they buried its head in the backyard of their house and dumped the body in the river."

The International Union for the Conservation of Nature Bornean (IUCN) lists Bornean and Sumatran orangutans critically endangered, blaming the loss of their natural habitat, human pressure and climate change for the dwindling population.

The World Wildlife Fund (WWF) estimates there are around 120,000 orangutans left in the wild.