Some patients claim to have put on weight during Jammeh's treatment

"I'm astonished. The danger of a president standing up [to say this] is shocking," Jerry Coovadia told the BBC.

Mr Jammeh said last month he had begun treating 10 patients on Thursdays with secret medicinal herb ingredients.

His health minister backs his claims, saying in trials so far patients had gained weight and physically improved.

"A response within three to 10 days and a three-day course is almost inconceivable for a disease like HIV/Aids," said Prof Coovadia, who heads the HIV research team at the University of KwaZulu Natal and is a member of South Africa's Treatment Action Campaign.

He said that science was many years away from finding a cure "so the fact that someone announces a cure like this is exceedingly difficult to accept".

'Confidence'

President Jammeh, who says he can also cure asthma, made his announcement to a gathering of foreign diplomats last month.

I've noticed I've increased weight substantially over the last 10 days

Patient Ousman Sowe

"I can treat asthma and HIV/Aids... Within three days the person should be tested again and I can tell you that he/she will be negative," he said in a statement.

"I am not a witch doctor and in fact you cannot have a witch doctor. You are either a witch or a doctor."

Gambian Health Minister Tamsir Mbow says the herbal medicines are taken orally and applied to the body.

"We cannot actually tell you the type of herbs we are using presently, it will be known to the whole world later on," Dr Mbow told the BBC.

One of the patients currently undergoing the treatment is Gambian university lecturer Ousman Sowe.

"I've noticed I've increased weight substantially over the last 10 days. I am no longer suffering from constipation, but we have yet to receive result of the tests," he told the BBC.

"I have 100% confidence in the president and I'm taking the medication with all confidence."

Risky behaviour

But Mr Coovadia said it was tragic that The Gambia had a "political environment that allows a minister of health and a president to violate every foundation of science and public health."

"The entire exercise is circumscribed by secrecy - that's not how science works," he said.

It would be impossible to measure the negative impact of Mr Jammeh's claims, but it could lead to risky sexual behaviour, instead of following preventative advice, he said.

The World Health Organisation told the BBC it did not wish to comment on the issue at this stage.

Last year, South Africa's health minister came in for severe criticism for promoting a diet of garlic and beetroot to those with HIV, while not rolling out the anti-retroviral drugs which are the only recognised treatment.

South Africa has now reversed its controversial advice.

To hear the full interview with Jerry Coovadia and more on Mr Jammeh's alleged healing powers tune in to the BBC World Service's Weekend Network Africa on Saturday 3 February 2007.