The author of a new book on the relationship between Israel and South Africa has accused Israeli President Shimon Peres of evading the question of whether he offered to sell nuclear weapons to the apartheid regime in the 1970s.
"If Peres denies he made such an offer, perhaps he would like to explain exactly what he did discuss at these meetings," said American academic Sasha Polakow-Suransky, author of The Unspoken Alliance: Israel's secret alliance with apartheid South Africa, published this week in the US.
Dr Polakow-Suransky was referring to a series of meetings in 1975 in which - according to "top secret" minutes he has uncovered - the South African defence minister, P W Botha, asked Israel for warheads capable of carrying nuclear weapons. The documents seem to show Mr Peres, then the Israeli defence minister, responding by listing weapons available for sale.
However, on Monday, the Guardian led with a story claiming that Mr Peres had formally offered to sell South Africa nuclear weapons. When Mr Peres said that the "correct payload" for the warheads was "available in three sizes", said the paper, this was "believed to refer to the conventional, chemical and nuclear weapons".
Sources close to President Peres said that he was "astounded" by the claims in the Guardian and stressed that "nothing in the documents published pointed in any way to Peres offering the South Africans nuclear weapons. What he said could be explained in many different and much more plausible ways."
Other sources have speculated that the "three sizes" Mr Peres allegedly referred to concerned the weights of the payloads rather than types of weapons.
The president's office also attacked the Guardian for publishing its story without first asking Mr Peres for his response. His press adviser, Ayelet Frisch, wrote a stern letter to editor Alan Rusbridger protesting that the president had not been given a basic right of reply.
Former South African foreign minister Pik Botha and former South African president F W de Klerk also denied the allegations, saying they had never been informed of any nuclear co-operation between Israel and South Africa.
"The allegations seem to be most unlikely," added Mr de Klerk. "Countries simply don't offer one another nuclear weapons like muffins at a tea party (in three different sizes!). Any such offer would have entailed enormous and unacceptable risks for Israel - particularly in the political climate of the time. What could South Africa possibly offer in return? And if such an offer had been made, why would South Africa have turned it down and proceeded to produce its own weapons at such great expense?"
The Guardian claimed that the minutes were the first official documentary evidence that Israel possessed nuclear weapons. Israel maintains a state of "nuclear ambiguity", in which it neither confirms nor denies the existence of a nuclear weapons arsenal.
The timing of the allegations was particularly sensitive, as the month-long 2010 Review Conference of the Parties to the Treaty on the Non-Proliferation of Nuclear Weapons concludes today at United Nations headquarters in New York. In a break from tradition, early drafts of the conference's final document called upon non-signatory countries such as Israel to join.
Asked how he thought the Guardian story would impact on the conference, Dr Polakow-Suransky said: "I imagine the revelations will be used as fodder by other nations to denounce Israel.
"The irony, of course, is that my research confirms that nuclear aspirants who near the threshold of weapons capability will do everything in their power to cross the finishing line. The historical case studies of Israel's and South Africa's programmes demonstrate that when states get close they go underground if necessary to finish the project, regardless of international suspicions and pressure.
"I am not an expert on Iran but I would not be surprised if they have studied the Israeli and South African examples of deception and covert development."