Bornean orangutan File picture: David W Cerny/Reuters

Jakarta - A critically endangered Bornean orangutan found dead in Indonesia's Central Kalimantan province on Monday died three days earlier from air rifle wounds before it was decapitated, Indonesia's Centre for Orangutan Protection said.

Authorities previously said the great ape appeared to have been stabbed multiple times in the latest in a string of unnatural orangutan deaths on the island to which they are native.

A necropsy carried out by police and the centre found 17 air rifle bullets in the male orangutan corpse, "proving" it died because of humans, said Ramadhani, the center's habitat protection manager.

"Our strong suspicion is that this orangutan died because it was shot using an air rifle through the heart, lungs and stomach," he said in a statement on the center's website.

The World Wildlife Fund estimates there are 104 700 Bornean orangutans, known for their broad faces and dark brown fur, left in the world.

Indonesia has vast swathes of plantations growing palm oil or coffee on deforested land that conservationists say often encroach on or disrupt ecosystems. Plantation workers sometimes encounter wildlife and may capture or kill the animals.

Data from the Centre for Orangutan Protection shows more than 15 of the creatures had been killed by humans or found dead in unnatural circumstances since 2011.

An investigation into the latest case is a test of the bravery of the Environment and Forestry Ministry, Ramadhani said.