Baby primate Vena shyly turned her head away from a bottle as two vets tried to feed her, the latest Bornean orangutan rescued in Indonesia after being kept as a pet.

Villagers on the Indonesian part of jungle-clad Borneo island often keep the critically endangered apes as pets even though the practice is illegal.

Wildlife officials and environmentalists rescued seven-month-old Vena earlier in February from someone in Kendawangan district who had been looking after her.

Vena is now being cared for at a centre run by NGO International Animal Rescue (IAR), whose staff ensure she stays clean by regularly changing her diapers and feed her bottles of milk mixed with vitamin supplements.

Last year IAR saved 22 orangutans that were either kept as pets or whose natural jungle habitat had been destroyed by huge forest fires started to clear land for plantations.

Even when they are well looked after, such as in Vena's case, environmentalists stress keeping orangutans as pets is bad because it means they will later struggle to survive in the wild.

"Many people don't realise that keeping orangutans as pets is illegal and could make them lose their instincts for living in the wild," said Ruswanto, an official from the wildlife protection agency who like many Indonesians goes by one name.

Vena was being kept as a pet by a lady called Bariah, who found the ape in a neighbouring village. She was rescued after villagers reported the case to authorities.

It was the second time Bariah, a mother of seven, was caught illegally caring for a baby ape -- she already had to give one up to IAR in 2016.

"I know orangutans are protected, I was not killing or harming them, I was only taking care of them," the 50-year-old told AFP.

After being rescued, young apes are sent to a "jungle school", where they spend years learning to fend for themselves before being released into the wild.

Rampant logging and the rapid expansion of paper and palm oil operations have reduced their habitat, with about 100,000 estimated to remain in the wild on Borneo, which is divided between Indonesia, Malaysia and Brunei.

The International Union for the Conservation of Nature last year changed its classification of the Bornean orangutan from "endangered" to "critically endangered".