US Republican presidential candidate, Mitt Romney (L) with Israel PM, Benjamin Netanyahu
US Republican presidential candidate, Mitt Romney would respect an Israeli decision to use military force to stop Iran from developing a nuclear weapon, one of his aides has said.
Romney, who is in Jerusalem, is expected to pledge closer ties between the US and Israel if he is elected.
US President Barack Obama has focused on using sanctions to contain Iran's nuclear ambitions.
The first leg of Romney's trip, in London, was marred by controversy.
After talking of "disconcerting" signs in London's preparations for the Olympic Games, Romney backtracked and predicted a "very successful" Olympics.
On Sunday morning Romney held talks with Israeli Prime Minister Benjamin Netanyahu and President Shimon Peres.
He told Peres he shared Israel's concern about the development of Iran's nuclear capabilities, saying: "The threat it would pose to Israel, the region and the world is incomparable and unacceptable."
Romney will give a speech later on Sunday near Jerusalem's Old City in which he is expected to say it is "unacceptable" for Iran to have the "capacity" to develop nuclear weapons.
"If Israel has to take action on its own, in order to stop Iran from developing the capability, the governor (Mr Romney) would respect that decision," his foreign policy adviser, Dan Senor told reporters ahead of the speech.
After his meetings with Israeli officials, he went to Jerusalem's Western Wall, one of Judaism's holy sites.
Romney will be hoping that burnishing his pro-Israel credentials will help him among key constituencies in a tight race with Obama, analysts say.
Romney says Obama has undermined Israel and supported its enemies.
The Republican presidential hopeful is also scheduled to meet Palestinian Prime Minister Salam Fayyad, though not Palestinian President Mahmoud Abbas.
While not explicitly ruling out military intervention, President Barack Obama's policy has emphasised non-military means of putting pressure on Iran.
The BBC's North America editor, Mark Mardell says Romney is highly critical of the international talks taking place which might lead to Iran being allowed to enrich some uranium. Romney wants zero enrichment.
In his speech on foreign policy, Romney will say he hopes the military option on Iran can be avoided but that it should not be taken off the table.
Romney says this is the best chance of focusing the minds of Iranian leaders on finding a peaceful solution.
A source in Romney's campaign said he also agreed with those who worried the Arab spring could turn into an "Islamist winter", our correspondent says.