A critically endangered Sumatran elephant has been found decapitated with its tusks missing in Indonesia, the conservation agency said Monday, as it opened a poaching investigation.

The rotting carcass was discovered Sunday at a palm oil plantation in Sumatra's Aceh region and a subsequent autopsy found that the animal had also been poisoned.

"The remains of the wild elephant were found in a tragic condition with its head gone," said Rosa Rika Wahyuni, a doctor at the Aceh conservation agency.

"We found poison in its digestive system," she added.

It was not clear how long the male elephant, estimated to be about 12 years old, had been dead.

Rampant deforestation has reduced the elephants' natural habitat and brought them into increasing conflict with humans, while their tusks are prized in the illegal wildlife trade. 

There have been several elephant poisoning cases in recent years, including one in 2019 when a Sumatran elephant was found decapitated with its tusks ripped off.

Aceh's conservation agency estimated the region has as few as 500 Sumatran elephants still living in the wild.