Two critically endangered Sumatran elephants were found dead in an Indonesian national park and it is believed they were poisoned, the WWF environmental group said Monday.

It takes to three the number of the elephants found dead in Tesso Nilo National Park on Sumatra island in the last month.

The carcasses of a male aged around five and a young female were found on Friday about a kilometre (0.6 miles) apart, said WWF spokeswoman Syamsidar, who goes by one name.

"We believe that the elephants were poisoned as the carcasses were quite close to each other," she said, adding that autopsies needed to be conducted before the cause of death could be confirmed.

A Sumatran elephant was discovered dead in the park early last month, also from suspected poisoning, she added.

Fifteen Sumatran elephants were found dead last year in Riau province, where the national park is located, with around half them found to have been poisoned, Syamsidar said.

Fewer than 3,000 Sumatran elephants remain in the wild, according to the International Union for Conservation of Nature.

Rampant expansion of palm oil and paper plantations and the mining industry have destroyed nearly 70 percent of the elephant's forest habitat over 25 years, according to the WWF, and the animals have been targeted by poachers.

In January 14 Borneo pygmy elephants were found dead of suspected poisoning in the Malaysian state of Sabah. Three-month-old orphaned calf Joe made headlines around the world when he was pictured trying to rouse his dead mother.