President Thabo Mbeki's telephone conversation with a discredited American Aids dissident has angered Aids scientists around the world and cast deeper shadows on the government's faltering Aids strategy only months before the world's most important annual Aids conference takes place in Durban in July.

"Mbeki has given lifeblood to a dead cause," said Dr John Moore of the Aaron Diamond Aids research centre in New York, affiliated to Rockefeller University.

David Rasnick, a San-Francisco-based microbiologist, said he had a 10-minute phone conversation with Mbeki on January 21 in which Mbeki asked if he would "support his efforts regarding AZT and Aids".

Mbeki called Rasnick after the American had replied to some questions on Aids faxed to him from the president's office.

Rasnick and his colleagues Peter Duesberg and Charles Geshekter are proponents of discredited theories on HIV, including the notion that the virus does not cause Aids, that Aids in underdeveloped countries is just another name for fatal diseases aggravated by a lack of sanitation and malnutrition, and that anti-HIV drugs are toxic and may even cause Aids. Parks Mankahlana, Mbeki's spokesperson, was not available for comment at the time of going to press.

Mbeki's apparent interest in disproving the wealth of medical knowledge on Aids has frustrated Aids activists, who claim his interventions are setting back an already inadequate effort against Aids.

"It's a national scandal," Malegapuru Makgoba, the president of the Medical Research Council, said of Mbeki's continuing flirtation with dissident Aids theories.

"Somebody here has to decide that the dissident group is wrong or right, and the only way you make this decision is if the dissidents have ever provided a theory or hypothesis that is testable. The answer is no."

Makgoba said the questions Mbeki had asked of Rasnick were "trivial" and "blew the mind". He described the issue as political rather than scientific.

"One of the things that distinguishes politics from science is that in science we never seek consensus.

"In science you are either right or wrong," he said. "If politicians are seeking consensus among scientists, that's the wrong approach," Makgoba said.

In September 1995 the US National Institutes of Health published a 61-page document refuting the dissident theories point-for-point, Makgoba said.

Researcher Moore, in a telephone interview on Friday, said he was "flabbergasted" and that the matter would be brought to the attention of "very serious levels in the US government. needs to get proper advice, from his peers."

"It's tantamount to Holocaust denial because the implications are so serious. You should not try to steer government policy on a path that could lead to the genocide of a nation," Moore said."

"To see these questions resurging in a country where the Aids problem is so much more serious is shocking and frightening, and to see the president of a nation taking this seriously is a very shocking thing," said Moore.

The link between HIV and Aids, and the effectiveness of anti-retroviral drug AZT, whose safety Mbeki has also been questioning, were "non-issues, for all practical purposes in America", Moore said.

"Everybody in Aids research who ever logs onto e-mail is talking about this," he said, and were "deeply concerned".

"There is no debate amongst scientists that HIV causes Aids," said Lynn Morris, the head of the Aids unit at the National Institute for Virology, which is part of South Africa's department of health.

"This debate is being generated by people on the fringe, in the lay press, and not by people who are actively involved in HIV research."

Dr Art Amman, a paediatrician and head of the US-based Global Strategies for HIV Prevention, said in a group email to international colleagues this week: "After reviewing the volumes of communication having to do with the Duesberg disciples, personally listening in court for two days to these individuals, and surveying the damage they are invoking, I am trying to reach some conclusions and think about a rational approach to limiting their future damage and influence."

Amman's -mail said the scientific community should publish "a denunciation of these individuals and their theories as not credible, dangerous and analogous to other pseudo-scientific theories in the past which are taken up by despots for nefarious intent," such as theories of eugenics and racial superiority.

"I find it curious there has been so little objection from black African or US leadership about a white-dominated movement like the Duesberg Disciples, which is perpetuating a theory that is resulting in the death of so many Africans," he added.

Moore, responding to suggestions that it was Mbeki's "democratic right" to seek further information, said: "It's the South African government's right to reinvent the wheel if they want, but these debates have been held, had and settled in America and Europe 10 years ago."