Four Sumatran elephants have been found dead in Indonesia's Aceh province, a local environmental group official said Saturday.

Corpses showed signs of poisoning from substances believed to be glued onto palm oil trees that elephants feed on, Fakta organization chief Wiranata said.

"We are currently investigating the incident and trying to find out the poisoning motive - did those elephant wreck farmers land or was it ivory hunters?" he said.

Elephant corpses were found in two locations two days ago, he said. Villagers said the ivory was intact on one animal estimated to be around 4 years old.

On April 30, a female Sumatran elephant was found dead in a palm oil plantation in Aceh Jaya district.

Conservationists said conflicts between humans and animals are increasing due to clearing of forests for plantations.

The Indonesian Elephant Conservation Forum said there were between 2,400 and 2,800 of the animals remaining in the wild, 500 of them in Aceh on the northern tip of Sumatra island.

In January, the environmental group World Wide Fund for Nature warned that the Sumatran elephant could be extinct in less than 30 years unless the destruction of their habitat is halted.

In December, the International Union for Conservation of Nature, changed the status of the sub-species from endangered to critically endangered.

The Sumatran elephant is a sub-species of the Asian elephant.