Authorities are investigating the unexplained deaths over the past months.

Poaching has been ruled out as the carcasses were found intact.

"Three laboratories in Zimbabwe, South Africa and Canada have been identified to process the samples taken from the dead elephants," the Ministry of Environment, Natural Resources, Conservation and Tourism said in a statement.

Elephants Without Borders (EWB), a conservation organisation, reported that its own aerial surveys showed that elephants of all ages appeared to be dying.

The group counted 169 dead elephants on May 25 and another 187 on June 14.

"Several live elephants that we observed appeared to be weak, lethargic and emaciated. Some elephants appeared disorientated, had difficulty walking, showed signs of partial paralysis or a limp," EWB director Mike Chase said in a report seen by Reuters.

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

In the report, EWB put the death toll at 356 and Chase said urgent action was needed to establish if the deaths were caused by disease or poisoning.

Africa's overall elephant population is declining due to poaching but Botswana, home to almost a third of the continent's elephants, has had numbers grow to 130,000 from 80,000 in the late 1990s.

However, they are seen as a nuisance by some farmers, whose crops have been destroyed.

Australian Associated Press