Shimon Peres says Israel and its allies must retain a firm stance on Iran until it can show it does not threaten regional stability, despite its election of Hassan Rouhani as president.
In an interview published Monday by the Brazilian newspaper the Folha de Sao Paulo, the Israeli president poured cold water on Rouhani's recent charm offensive.
Last month, Rouhani sought to strike a new more open tone at the UN General Assembly, in stark contrast to the aggressive rhetoric of his predecessor Mahmoud Ahmadinejad.
He envisaged signing a nuclear deal with world powers within months, raising hopes in Western capitals, which suspect Iran's research program is a cover for developing an atomic bomb.
"In politics and in life you can only judge things based on facts. There has been no change on Iran -- the facts contradict the speeches," Israel's elder statesman told the paper.
"If the Iranians say they do not want nuclear bombs then why are they developing missiles?"
Iran insists its nuclear program is solely for peaceful purposes.
Peres has not endorsed unilateral Israeli military action against Tehran to curb its nuclear ambitions, but says diplomatic pressure must be ramped up.
This, the 90-year-old said, would mean Russia weighing in.
"We all prefer a non-military solution. But in order to give credibility to non-military solutions Iran has to know it will stand alone and could face force," he said.
"There are coalitions of countries -- one group is led by the United States and the European Union. If the Russians were on this side the pressure would be much more effective."
Asked what he would say to Rouhani if the two were to pick up the telephone and speak directly,
Peres said: "I would tell him nobody in the world is threatening Iran. So why does Iran threaten other countries? Tell me. I don't understand why Iran threatens Israel. Why?"