WASHINGTON // The US Defence secretary believes Iran is laying the groundwork for making nuclear weapons but is not yet building a bomb.
Leon Panetta called for continued diplomatic and economic pressure to persuade Tehran not to take that step.
As he has done previously, Mr Panetta cautioned against a unilateral strike by Israel against Iran's nuclear facilities, saying the action could trigger Iranian retaliation against US forces in the region.
"We have common cause here" with Israel, he said. "The better approach is for us to work together."
Mr Panetta's remarks, on CBS's Face the Nation show, which were taped on Friday and aired yesterday, reflect the long-held view of the Obama administration that Iran is not yet committed to building a nuclear arsenal, only to creating the industrial and scientific capacity to allow one if its leaders decide to take that final step.
The comments suggest the White House's assessment of Iran's nuclear strategy has not changed in recent months, despite warnings from advocates of military action that time is running out to prevent Tehran from becoming a nuclear-armed state.
Several Republican candidates have called for a tougher line against Iran, saying they believe it is committed to building a bomb.
Iran has opened two dozen of its facilities to international inspectors but has refused in defiance of the UN Security Council to suspend its uranium enrichment.
In a talk at a Brookings Institution forum in December, Mr Panetta said an attack on Iran's nuclear facilities would "at best" delay Iran's nuclear programme by one or two years.
Among the unintended consequences, he said, would be an increase in international support for Iran and the likelihood of Iranian retaliation against US forces and bases in the Middle East.
Mr Panetta did not discuss the issue directly on Face the Nation.
But the chairman of the joint chiefs of staff, Gen Martin Dempsey, appearing with the defence secretary, said he wanted the Iranians to believe that a US military strike could wipe out their nuclear programme.
"I absolutely want them to believe that is the case," he said.
Mr Panetta did not rule out launching a pre-emptive strike.
"But the responsible thing to do right now is to keep putting diplomatic and economic pressure on them to force them to do the right thing," he said. "And to make sure that they do not make the decision to proceed with the development of a nuclear weapon."
Mr Panetta said that if Iran started developing a weapon, the US would act.
"They need to know that - that if they take that step, that they Are going to get stopped."
Gen Dempsey also said that Iran has the military power to block the Strait of Hormuz "for a period of time" but that the US would take action to reopen it. "We can defeat that," he said.