The advice comes at a time when at least three African heads of state have said they are importing large quantities of a herbal tonic which the president of Madagascar claims can cure the virus.

But its efficacy has not been proven by scientific peer reviews, and even Madagascar's own national medical academy has cast doubt on it.

In a statement, the WHO says it welcomes the use of traditional medicines in the search for potential treatments but it warns that they must first be robustly tested.

Madagascar's President Andry Rajoelina launched a product based on the artemisia plant last month when it had been tested on fewer than 20 people.

He says clinical trials will start next week.