Israeli Prime Minister Benjamin Netanyahu said on Sunday that international sanctions have so far made no impact on Iran's nuclear programme and that a "strong and credible" military threat was needed.
"We have to be honest and say that all the sanctions and diplomacy so far have not set back the Iranian programme by one iota," he said on meeting White House hopeful Mitt Romney.
"I believe that we need a strong and credible military threat, coupled with the sanctions, to have a chance to change that situation."
His remarks came as the Haaretz newspaper ran a story saying that US National Security Adviser Tom Donilon had recently briefed Netanyahu on Washington's contingency plans for a pre-emptive strike on Iran's nuclear facilities.
Citing a "senior American official," Haaretz said Donilon had held a three-hour dinner meeting with the Israeli premier a fortnight ago, during which he had shared details about US military capabilities for attacking underground Iranian bunkers.
It said Donilon's Israeli counterpart, Yaakov Amidror, was present "for part of the time."
But an Israeli official, speaking on condition of anonymity, denied the report.
"We don't comment on what is discussed in closed diplomatic meetings," the official told AFP. "But the story is full of factual errors.
"Nothing in the article is correct.
"Donilon did not meet the prime minister for dinner, he did not meet him one-on-one, nor did Donilon present operational plans to attack Iran."
Haaretz wrote: "Donilon sought to make clear that the United States is seriously preparing for the possibility that negotiations will reach a dead end and military action will become necessary."
In recent weeks, several senior US officials have held talks in Jerusalem, among them US Secretary of State Hillary Clinton, Obama's top counter-terrorism adviser John Brennan and Clinton's deputy William Burns.
And US defence chief Leon Panetta is due in Israel next week for top-level talks, with Iran likely to play a central role in his discussions.
Netanyahu told Romney he shared the Republican challenger's position that a nuclear Iran was "the greatest danger facing the world."