U.S. Defense Secretary Leon Panetta let slip on Sunday the big open secret that Washington war hawks don’t want widely known: Iran is not developing nuclear weapons.

Appearing on CBS’s Face the Nation on Sunday, Panetta admitted that despite all the rhetoric, Iran is not pursuing the ability to split atoms with weapons, saying it is instead pursuing “a nuclear capability.”

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That “capability” falls in line with what Iran has said for years: that it is developing nuclear energy facilities, not nuclear weapons.

“I think the pressure of the sanctions, the diplomatic pressures from everywhere, Europe, the United States, elsewhere, it’s working to put pressure on them,” Panetta explained on Sunday. “To make them understand that they cannot continue to do what they’re doing. Are they trying to develop a nuclear weapon? No. But we know that they’re trying to develop a nuclear capability, and that’s what concerns us. And our red line to Iran is, do not develop a nuclear weapon. That’s a red line for us.”

Republicans have been beating the drums of war in recent weeks as tensions in the Iranian gulf have soared. Iran has threatened to shut down the Strait of Hormuz, a key oil transport hub crucial to global industry, if U.S. warships return to monitor their activities.

Iran said it was planning to hold military exercises in the Strait of Hormuz in the coming weeks, and prior wargames saw the Iranians test missiles that are designed to sink warships.

President Barack Obama recently agreed to fresh sanctions on Iran targeting the country’s central bank, in hopes of slowing down their nuclear program. The European Union was also considering fresh sanctions, and details were expected later in January. The U.N., as well, has sanctioned Iran repeatedly over its nuclear program.

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Iran said recently that it had created the country’s first ever nuclear fuel rod made from domestic uranium enriched at their own facilities.

Nuclear fuel enrichment is much different from enrichment for weapons. Most commercial nuclear reactors use lightly enriched uranium, which is between 3-5 percent enriched. Weapons-grade uranium must be enriched to approximately 85 percent or more of a key radioactive isotope for it to be usable in an atomic bomb.

Iran added on Monday that it had also enriched uranium up to 20 percent in an underground facility, explaining that the isotopes were to be used to help cancer patients.

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The International Atomic Energy Agency said late last year that Iran had carried out tests that suggested they may be taking the first steps toward building a nuclear weapon, but former agency insiders disputed the claim as being misleading.

This video is from CBS News, broadcast Sunday, Jan. 8, 2012, as clipped by ThinkProgress.