Hundreds of elephants that died mysteriously in Botswana's famed Okavango Delta probably succumbed to natural toxins, the wildlife department said Friday.

​The landlocked southern African country has the world's largest elephant population, estimated to be around 130,000. Around 300 of them have been found dying since March.

​Authorities have so far ruled out anthrax, as well as poaching, as the tusks were found intact.

​Preliminary tests conducted in various countries far have not been fully conclusive and more are being carried out, Wildlife and Parks Department boss Cyril Taolo told AFP in a phone interview.

​"But based on some of the preliminary results that we have received, we are looking at naturally-occurring toxins as the potential cause," he said.

​"To date we have not established the conclusion as to what is the cause of the mortality".

​He explained that some bacteria can naturally produce poison, particularly in stagnant water.

​Government has so far established that 281 elephants died, although independent conservationists say more than 350.

​The deaths were first flagged by a wildlife conservation charity, Elephants Without Borders (EWB), whose confidential report referring to the 356 dead elephants was leaked to the media early in July.

​EWB suspected elephants had been dying in the area for about three months, and mortality was not restricted to age or gender.

​Several live elephants appeared weak, lethargic and emaciated, with some showing signs of disorientation, difficulty in walking or limping, EWB said.

​Tests are being conducted at specialist labs in South Africa, Canada, Zimbabwe and the US.

© Agence France-Presse