President Shimon Peres, considered the father of Israel's nuclear programme, on Monday sought to justify the policy of ambiguity his country has adopted concerning the issue.

On Iran, Peres said newly elected President Hassan Rouhani represented a "chance", in contrast with Israeli Prime Minister Benjamin Netanyahu's insistence the moderate leader was a "wolf in sheep's clothing".

"Its a chance, undecided yet," Peres, whose role is largely ceremonial, told the France 24 television network.

"Apparently there are differences within Iran (over its nuclear programme); who is going to win I don't know."

Foreign experts say Israel has at least 100 nuclear warheads, but the Jewish state -- which has not signed up to the Non-Proliferation Treaty -- has never confirmed nor denied this.

Peres defended Israel's ambiguity on its own nuclear capabilities, saying it was a policy that acted as a deterrent.

"It depends why you want nuclear weapons. For me a nuclear option was an instrument to lead to peace, not to attack," said the Israeli president.

"It was a deterrence that had nothing to do with reality whether we have it or not.

"Fantasy plays an extremely great role to increase the deterrence, so it is ambiguous, yes. Why should I clarify?"

For more than 40 years, successive Israeli governments have said their country would not be the first to introduce nuclear weapons to the Middle East.

Experts say Israel entered into an agreement in 1969 with the United States under which it must refrain from making public its nuclear potential and any atomic test.

In return, Washington has pledged not to pressure it on the issue.

Israeli leaders have yet to break the taboo by acknowledging the existence of a nuclear arsenal, although they have become more transparent in recent years, notably Peres.

Peres is considered the main architect of the Israeli nuclear programme, launched in close cooperation with France in the late 1950s.