Hundreds of elephants have died mysteriously in Botswana's Okavango Delta, the head of the wildlife department said, ruling out poaching as the tusks were found intact.

The country has the world's largest elephant population, estimated to be around 130,000.

"We have had a report of 356 dead elephants in the area north of the Okavango Delta and we have confirmed 275 so far," Cyril Taolo, the acting director of the department of Wildlife and National Parks said.

He said the cause of the deaths was yet to be established with anthrax having been ruled out.

"We do not suspect poaching since (the) animals were found with tusks," he said.

The Director of Conservation at National Park Rescue - which works to protect national parks in Africa - said he cannot think of a single example in which so many elephants died in such a short space of time for "a completely unknown reason".

Speaking on RTÉ's News at One, Dr Niall McCann said airplanes have captured images of some elephants dying "incredibly quickly", while others are wandering around in a "confused manner with obvious motor function issues".

This suggests something is affecting their neurological systems, he said, and that he would guess it is either a poison or a pathogen.

"It’s hard to rule out either one at this stage. Even if it is a poison, it might be a natural occurring one. But it might also have been laid by poachers."

While the elephants’ tusks are still intact, Dr McCann said the lure of over 800 tusks "lying out there" must be "a magnet" to poachers.

Test results could take "two to three weeks" to come back, he said, by which time more elephants will likely die.

Samples have been collected and sent to South Africa, Zimbabwe and Canada for testing.

Similar deaths were first reported in May when authorities found 12 carcasses in just a week in two villages in the northwest of the country. 

The latest discoveries were flagged by a wildlife conservation charity, Elephants Without Borders (EWB), whose confidential report referring to the 356 dead elephants, was leaked to the media yesterday.

EWB suspects the elephants have been dying in the area for about three months.

According to the report dated 19 June, "70% of elephant carcasses were considered recent, having died about a month ago, and 30% of the carcasses appeared fresh, ranging from one day to two weeks old".

"There was good evidence to show elephants of all ages and sex appear to be dying," said the report compiled by EWB director Mike Chase.

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

"One elephant was observed walking in circles, unable to change direction although being encouraged by other herd members," said the report.