The deaths of the endangered animals, found last month in a forest reserve in Sabah state on Borneo island, shocked conservationists and officials, who released a poignant picture of a three-month-old calf nuzzling its dead mother.

Masidi Manjun, state tourism, culture and environment minister, said preliminary results of chemical analysis by government scientists had not provided conclusive evidence on the cause of death.

Wildlife officials would send samples to two forensic testing facilities in Thailand and Australia in an effort to "find any chemical compounds that may have been the cause of the elephants' death", he said in a statement.

Officials suspect the elephants may have been poisoned due to severe ulceration and bleeding in their digestive tracts.

Substances left out by workers at nearby plantations to deter them from eating the palm fruit may have been to blame, they believe.

Masidi said that police were conducting "a very thorough investigation involving all possible parties, including plantations and logging companies within the vicinity where the dead elephants were found".

Malaysian authorities have said they will offer a $16,000 reward for information on the case if it is confirmed the elephants were poisoned, and that the culprits would face stiff penalties.

Officials are still trying to save the calf found with the dead animals, now staying in a wildlife park in Sabah.

WWF-Malaysia says only about 1,200 Borneo pygmy elephants, which are smaller and have more rounded features than full-sized Asian elephants, are estimated to be left in the wild.

Their habitat is shrinking due to logging for timber, oil palm plantations and other development, forcing them to find alternative food and space and putting them in conflict with humans.