U.S. Defense Secretary Leon Panetta outlined red lines for Iran in a TV interview on Sunday, vowing that the United States would not allow Iran to develop a nuclear weapon or to block the Strait of Hormuz, as Iran began uranium enrichment at a new underground site well protected from possible airstrikes.

"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," Panetta said in the interview on the "Face the Nation" program of CBS. "And our red line to Iran is to not develop a nuclear weapon. That's a red line for us."

Panetta also said that Washington would "not tolerate" the blocking of the Strait of Hormuz, saying "that's another red line for us and that we will respond to them."

Appearing in the same interview, General Martin Dempsey, the chairman of the U.S. Joint Chiefs of Staff, added that while the Iranians have the capability to block the strait for a time, the United States would also be able to "defeat" them if it happened.

"They've invested in capabilities that could, in fact, for a period of time block the Straits of Hormuz. We've invested in capabilities to ensure that if that happens, we can defeat that," he said. "We've described that as an intolerable act. And it's not just intolerable for us, it's intolerable to the world. But we would take action and reopen the Straits."

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

Iran has insisted its nuclear program is for civilian and medical use only, but Western powers have accused it of harboring ambitions for nuclear weapons.