White House hopeful Mitt Romney on Sunday held top-level talks in Israel about how to handle fears over Iran's nuclear ambitions, on a visit aimed at burnishing his foreign policy credentials.

"Like you, we are very concerned about the development of nuclear capabilities on the part of Iran and feel it is unacceptable for Iran to become a nuclear armed nation," Romney told reporters on meeting President Shimon Peres.

"The threat it would pose to Israel, the region and the world is incomparable and unacceptable."

The Republican challenger, who will face off against President Barack Obama in November's US election, flew in from Britain late on Saturday for a one-day visit expected to focus on Iran's nuclear programme, which Israel and much of the West believes is a covert bid to develop atomic weapons.

"Iran and its effort to become a nuclear-capable nation (is one) which I take with great seriousness, and look forward to chatting with you about further actions that we can take to dissuade Iran from their nuclear folly," Romney told Prime Minister Benjamin Netanyahu earlier on Sunday in remarks carried on Israeli public radio.

Netanyahu told him 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."

"It's important to do everything in our power to prevent the Ayatollahs from possessing that capability," he said. "That's why I believe we need a strong and credible military threat, coupled with the sanctions, to have a chance to change that situation."

Israel is widely believed to have the Middle East's only, albeit undeclared, nuclear arsenal.

Romney later met Shaul Mofaz, leader of the Israeli opposition, who warned him it was not the right time for a military strike on Iran.

"We must be ready for any option, but the time for military action has not yet arrived," Mofaz said in remarks communicated by a spokesman.

"This is the time to deepen sanctions against the Iranian regime, to be alert to any development and in any case, to act in full coordination. A nuclear Iran is a global threat, and not just an issue pertaining to Israel," he said.

But Romney was expected to express support for Israel's right to mount a pre-emptive strike when he gives a foreign policy statement later on Sunday, the New York Times reported.

"If Israel has to take action on its own, in order to stop Iran from developing that capability, the governor would respect that decision," Dan Senor, one of Romney's senior foreign policy advisers, told reporters travelling with the delegation.

"It is not enough just to stop Iran from developing a nuclear programme. The capability, even if that capability is short of weaponisation, is a pathway to weaponisation, and the capability gives Iran the power it needs to wreak havoc in the region and around the world."

In his speech, Romney was to say: "Today, the regime in Iran is five years closer to developing nuclear weapons capability.

"Preventing that outcome must be our highest national security priority."

Romney will also meet Palestinian Prime Minister Salam Fayyad on Sunday.

Romney has consistently attacked what he says is Obama's weak and misguided Middle East policy, saying in January that the president "threw Israel under the bus," by defining the 1967 borders as a starting point for Israeli-Palestinian negotiations.

He also expressed concern "about the tragedy in Syria" and was "anxious to find a pathway to peace in Syria that would lead to greater stability and greater representation in the government in the interests of the majority and the minority populations of that nation."

On Egypt, Romney said he would work to ensure Cairo's new Islamist President Mohamed Morsi would protect the 1979 peace treaty with Israel.

"With the Islamist president elected in Egypt we hope to use the considerable weight of the world's influence to ensure a continued commitment to the agreements of peace.. with Israel."

In a show of support for Israel ahead of Romney's tour, Obama on Friday signed a law reinforcing US security and military cooperation with Israel as representatives of the pro-Israel lobby AIPAC stood beside him in the Oval Office.