The US military's top officer has warned that conflict in Iran could prove "extremely stressful" for his forces.
Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, said an Israeli strike on Iranian nuclear facilities would be a high-risk move that could destabilise the whole Middle East.
But he refused to elaborate on what Israeli leaders told him during meetings last week.
"I won't discuss the details of the concerns they expressed, nor will I comment one way or another about the speculation surrounding Israeli intentions," he said.
"Those are matters for the Israeli military and the Israeli government to address.
"But my position with regard to the Iranian regime hasn't changed. They remain a destabilising factor in the region, and that's evident and actually more evident when one visits."
Nevertheless, Adm Mullen went on to warn that any US intervention in Iran would stretch the army's capabilities, thanks to the nearly seven-year-old campaign in Afghanistan and more than five years in Iraq.
He said: "Opening up a third front right now would be extremely stressful on us.
"That doesn't mean we don't have capacity or reserve but that would really be very challenging and also the consequences of that are sometimes very difficult to predict."
He would not say if the US Navy was increasing its presence in that region but asserted that Iran would not have the ability to sustain a blockade of the strategic Straits of Hormuz in the event of any conflict.
The Bush administration and other world leaders claim Iran is seeking to produce nuclear weapons while Iran insists its nuclear programme is aimed only at generating electricity.
"I believe they're still on a path to get nuclear weapons and I think that's something that needs to be deterred," said Adm Mullen.
But he said this should be done through diplomatic, financial and economic actions by the US and other nations.
ITV.com