US Defence Secretary Leon Panetta has repeated his warning to Iran not to try and block the Strait of Hormuz, the main route for oil exports from the Gulf.
Iran is threatening to cut off the waterway if sanctions are imposed on its oil exports because of its nuclear ambitions.
The BBC reports America currently prefers non-military efforts to oppose Iran's nuclear ambitions. But in an interview on US television, Mr Panetta stressed a tougher stance if Iran tried to stop world oil shipments.
"We made very clear that the United States would not tolerate blocking of the Straits of Hormuz. That's another red line for us and we will respond to that."
Another red line to the Iranians would be any American nuclear weapons programme.
Mr Panetta reiterated the US assessment that Iran is laying the groundwork for making nuclear weapons one day, but not yet building a bomb.