A pregnant Sumatran elephant was found dead of suspected poisoning near a palm plantation in Indonesia's Riau province on the island of Sumatra, officials said on Friday.

Sumatran elephants are a critically endangered species and fewer than 700 remain on the island. The animal is protected under an Indonesian law on the conservation of biological natural resources and their ecosystems.

Local authorities are investigating the cause of death of the elephant, which was discovered in a joint patrol by local conservation groups on Wednesday. But they strongly suspect it was poisoned.

"From the sign of changes in the shape of its internal organs, such as the lung, it looks like it is burning, black and oozing from the blood," said Zulhusni Syukri, programme director of Rimba Satwa Foundation, one of the groups that found the dead animal.

The animal had blood coming out of its mouth and anus, according to Fifin Arfiana Jogasara, the acting head of Riau Natural Resources Conservation, another group on the patrol that found the animal.

The authorities also found pineapple in the elephant's stomach, even though pineapples are not grown in that area, further leading them to suspect poisoning, Syukri said.

The population of Sumatran elephants on the island shrank from 1,300 in 2014 to 693 last year, down nearly 50% in seven years, Indonesian forestry and environment ministry data show.

The decline has occurred amid a loss of more than 69% of the animal's potential habitat in the last 25 years - the equivalent of one generation.

The expansion of palm plantations in Sumatra has led to conflicts between humans and animals over space and resources.

"The palm plantation workers near the elephants' roaming paths often have conflicts with the elephants because they eat the palm fruit," Syukri said.

At least seven Sumatran elephants have died in Riau province in the last three years, he added.

The elephant found on Wednesday was estimated to be about 25 years old. During the necropsy, it was discovered that the elephant was pregnant and would soon give birth. The authorities buried the elephant's carcass at the site after taking samples of its liver, intestinal wall, lungs and faeces for laboratory testing.

Sumatran elephants are a subspecies of the Asian elephant.