Defense Secretary Leon Panetta said Sunday that the U.S. would act if Iran were to develop a nuclear weapon or close the Strait of Hormuz.

Panetta, speaking to CBS' Face the Nation, said that if Iran started developing a weapon, the U.S. would act. "I think they need to know that — that if they take that step — that they're going to get stopped."

Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey, appearing with the defense secretary, said that he wanted the Iranians to believe that a U.S. military strike could wipe out their nuclear program.

"I absolutely want them to believe that's the case," he said.

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."

Dempsey also said that Iran has the military power to block the Strait of Hormuz "for a period of time" if it decides to do so, but that the U.S. would take action to reopen waterway. "We can defeat that," he said.

Panetta said closing the strait would draw a U.S. military response. "We made very clear that the United States will not tolerate the blocking of the Strait of Hormuz," he said. "That's another red line for us and ... we will respond to them."

A number of experts say Iran is unlikely to close the strait, through which Gulf oil flows, because the action could hurt Iran as much as the West.

But an Iranian newspaper, the Khorasan daily, on Sunday quoted a senior commander of the powerful Revolutionary Guard force as saying Tehran's leadership has decided to order the closure of the strategic oil route if the country's petroleum exports are blocked.

Iranian politicians have issued similar threats in the past, but this is the strongest statement yet by a top commander in the security establishment.

In a talk at a Brookings Institution forum in December, Panetta said an attack on Iran's nuclear facilities would "at best" delay Iran's nuclear program 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 U.S. forces and bases in the Mideast.

Panetta also said Sunday that Iran is laying the groundwork for making nuclear weapons someday, but is not yet building a bomb.

As he has previously, Panetta cautioned against a unilateral strike by Israel against Iran's nuclear facilities, saying the action could trigger Iranian retaliation against U.S. forces in the region.

"We have common cause here" with Israel, he said. "And the better approach is for us to work together."

Panetta's remarks on Face the Nation, which were taped Friday and aired Sunday, 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 to 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.

Iran says its nuclear program is only for energy and medical research, and refuses to halt uranium enrichment

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.

A leading hardline Iranian newspaper reported Sunday that Iran has begun uranium enrichment at a new underground site well protected from possible airstrikes.

Kayhan daily, which is close to Iran's ruling clerics, said scientists have begun injecting uranium gas into sophisticated centrifuges at the Fordo facility near the holy city of Qom.