Indonesian villagers have beaten a Sumatran orangutan to death, an animal protection group said Tuesday, the latest case of one of the critically-endangered primates being killed by humans.

The adult female died Thursday after being rescued from a village in Aceh province with numerous injuries by the Sumatran Orangutan Conservation Programme.

Group director Ian Singleton said the primate was found with swelling to its head and body, a serious eye injury and bleeding under the skin around its jaw.

"The only way you would ever gain control of a wild adult orangutan is to beat and club it until it is barely conscious, or dead," he told AFP.

He said it was not clear why the animal was killed.

In some cases, people kill female orangutans when the apes are trying to stop their offspring being taken away to be sold as pets, he said, although in this case no baby was found.

Orangutans have also been attacked by workers on palm oil and paper plantations on their native Sumatra island who view them as pests.

Orangutans being killed by humans was "still a very common occurrence in Indonesia", he said.

Amon Zamora, the head of Aceh's conservation agency, said the authorities were investigating the case and it would take some time.

"Capturing orangutans for sale or as pets and harming them is certainly against the law," he told AFP.

Only around 7,300 Sumatran orangutans remain in the wild, according to protection group the International Union for Conservation of Nature.

Orangutans are faced with extinction from poaching and the rapid destruction of their forest habitat, driven largely by land clearance for palm oil and paper plantations.