The secret military agreement signed by Shimon Peres, now president of Israel, and P W Botha of South Africa. Photograph: Guardian

Secret South African documents reveal that Israel offered to sell nuclear warheads to the apartheid regime, providing the first official documentary evidence of the state's possession of nuclear weapons.

The "top secret" minutes of meetings between senior officials from the two countries in 1975 show that South Africa's defence minister, PW Botha, asked for the warheads and Shimon Peres, then Israel's defence minister and now its president, responded by offering them "in three sizes". The two men also signed a broad-ranging agreement governing military ties between the two countries that included a clause declaring that "the very existence of this agreement" was to remain secret.

The documents, uncovered by an American academic, Sasha Polakow-Suransky, in research for a book on the close relationship between the two countries, provide evidence that Israel has nuclear weapons despite its policy of "ambiguity" in neither confirming nor denying their existence.

The Israeli authorities tried to stop South Africa's post-apartheid government declassifying the documents at Polakow-Suransky's request and the revelations will be an embarrassment, particularly as this week's nuclear non-proliferation talks in New York focus on the Middle East.

They will also undermine Israel's attempts to suggest that, if it has nuclear weapons, it is a "responsible" power that would not misuse them, whereas countries such as Iran cannot be trusted.

A spokeswoman for Peres today said the report was baseless and there were "never any negotiations" between the two countries. She did not comment on the authenticity of the documents.

South African documents show that the apartheid-era military wanted the missiles as a deterrent and for potential strikes against neighbouring states.

The documents show both sides met on 31 March 1975. Polakow-Suransky writes in his book published in the US this week, The Unspoken Alliance: Israel's secret alliance with apartheid South Africa. At the talks Israeli officials "formally offered to sell South Africa some of the nuclear-capable Jericho missiles in its arsenal".