Documents unearthed by an American academic indicate Israel offered to sell nuclear weapons to South Africa in the 1970s.

Israel has never confirmed it has nuclear weapons despite a large amount of evidence that it has as many as 200 warheads.

The documents provide the first proof Israel not only has enough nuclear weapons for its own defence but has enough to sell elsewhere.

Like India, Pakistan and North Korea, Israel is not a signatory to the Nuclear Non-Proliferation Treaty.

In the 1980s, Mordechai Vanunu, a former technician who worked at Israel's main nuclear facility, revealed the extent of the country's arsenal.

Four years ago, former prime minister Ehud Olmert accidentally acknowledged the country's nuclear capability on German television.

And now an American academic, Sacha Polokow-Suransky, has unearthed documents which appear to indicate Israel offered to sell South Africa nuclear weapons during the era of apartheid.

The deal between Israel and South Africa did not proceed. But questions remain as to whether Israel helped South Africa develop nuclear weapons later on.

Dr Gil Merom, an international security expert from Sydney University, says Israel looked for alliances all over the world.

"Israel looked for any possible alliance and any possible support so South Africa could deliver to Israel mostly, I think, resources of the type of raw uranium," he said.

The claims come as 190 nations are meeting in New York at the Nuclear Non-proliferation Review Treaty.

Experts say they are likely to add more pressure on Israel to declare itself a nuclear power.

Dr Andrew Davies, an expert on nuclear issues from the Australian Strategic Policy Institute, says the news is unlikely to surprise anyone.

"Being found out as being a proliferator never helps anybody's case for taking the higher moral ground on nuclear weapons," he said.

"But I don't think any new revelation about Israel is going to come as a terrible surprise to anyone."

Israel has played a strong role in the effort to prevent Iran gaining nuclear weapons.

Dr Ron Huisken, an expert on nuclear weapons from the Australian National University, says the Iranians will use further proof Israel has nuclear weapons to justify its own program.

He also says the revelations could force the United States to ratchet up the pressure on Israel to admit having nuclear weapons.

"If Obama and co. were serious about a sustained process of nuclear diminution, an early part of that process has to be an Israel that comes out of the closet, and in the process of doing that, says yes, we've got the bomb," he said.

"We've got it for these reasons, and then begin to talk to its neighbours about the motives behind it and ... what Israel thinks the neighbours need to do for Israel to consider giving the capability up."

The NPT Review conference ends on Friday.