With Iran announcing major progress in enriching uranium, Washington threatened military action should Tehran try to develop a nuclear bomb or block oil exports in the Strait of Hormuz.
The US on Sunday issued a fresh warning to Iran over the country's nuclear ambitions and Tehran's threat to close the Strait of Hormuz, a vital international trading route for oil.
"We made very clear that the United States will not tolerate the blocking of the Straits of Hormuz," US Defense Secretary Leon Panetta told CBS television. "That's a red line for us and that we will respond to."
Iran had threatened to close the strait in response to the EU threatening further sanctions.
'Don't mess with that'
"I think the message that the world needs to understand is: America is the strongest military power and we intend to remain the strongest military power and nobody ought to mess with that," the defense secretary said.
Washington and the West accuse Tehran of trying to build an atomic bomb while Iran insists its nuclear program is only for peaceful energy production.
Nuclear talks in jeopardy
On Sunday, Tehran announced that its underground uranium enrichment facility would start operating soon.
"The Fordow nuclear enrichment plant will be operational in the near future," Fereydoun Abbasi Davani, head of the country's Atomic Energy Organization said.
If the Fordow site becomes operational, it could threaten fresh talks over Iran's nuclear ambitions. Tehran has called for new negotiations with the permanent members of the UN Security Council, which have been stalled for a year.
A red line
The United States and Israel have both refused to rule out military strikes against Iran should talks fail to resolve the dispute.
"We know that they're trying to develop a nuclear capability," Panetta said on Sunday. "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."
Panetta was seconded by General Martin Dempsey, chairman of the Joint Chiefs of Staff and therefore the US government's top military officer. "They need to know that if they take that step that they're going to get stopped," Dempsey told CBS.
Author: Andreas Illmer (Reuters, AFP)
Editor: Ben Knight