Sanctions are having a "serious impact" on the Iranian economy, even if their results may not be immediately obvious, US Defence Secretary Leon Panetta said on Monday.

Panetta, speaking in Tunisia at the start of a Middle East tour that will also take him to Egypt, Israel and Jordan, repeated the US government's insistence that all options were on the table to stop Tehran developing nuclear weapons.

"We recognise that these sanctions are having a serious impact in terms of the economy of Iran," he said.

"While the results of that may not be obvious at the moment, the fact is that they've expressed a willingness to try to negotiate (with the P5+1 powers) and they continue to seem interested in trying to find a diplomatic solution," he added.

The P5+1 group of nations are the five permanent members of the UN Security Council plus Germany that have been involved in negotiations to try to curb Iran's nuclear activities.

Panetta's comments come just a day after US President Barack Obama's Republican challenger Mitt Romney, during a trip to Jerusalem, backed Israel's right to thwart Iran's nuclear programme, which Tehran says has purely peaceful aims.

The Security Council has demanded Iran suspend all uranium enrichment and has imposed four sets of sanctions to pressure it to comply.

The United States and the European Union have added their own sanctions on Iran, but the Islamic republic has defiantly said it would continue with its nuclear activities.

"What we all need to do is to continue the pressure economically and diplomatically," said Panetta.

But "the United States will not tolerate an Iran that develops a nuclear weapon. We are prepared to exercise all options to ensure that does not happen.

"We have a full range of options in order to deal with that essential threat," the Pentagon chief said, without elaborating.

Meetings between Israeli and US officials have multiplied in recent months, with the Obama administration trying to persuade the Jewish state to give time for the sanctions to take effect.

Panetta is scheduled to travel to Jerusalem on Tuesday, via Cairo, for a visit during which he is expected to meet Israeli Prime Minister Benjamin Netanyahu.

Netanyahu told Romney, Obama's election rival, on Sunday that it was important to have "a strong and credible military threat" because sanctions and diplomacy "so far have not set back the Iranian programme by one iota."

Ahead of his arrival in Tunisia, Panetta said Obama had made clear that he respected the sovereignty of the Jewish state "and their ability to make decisions with regards to their own security."