US defence chief Panetta warns against Iran strike
The US Defence Secretary Leon Panetta has said a military strike against Iran could have "unintended consequences".
He said it would only delay Iran's nuclear efforts by three years at most.
Correspondents say the comments appear to play down speculation that a military strike might be used to cripple Tehran's nuclear programme.
On Tuesday, the UN's nuclear watchdog, the IAEA, said Iran was carrying out research aimed at developing nuclear weapons capacity.
"You've got to be careful of unintended consequences here," Mr Panetta told reporters in Washington, when asked about his concerns about a military strike.
He acknowledged military action might fail to deter Iran "from what they want to do".
"But more importantly, it could have a serious impact in the region, and it could have a serious impact on US forces in the region," he said.
"And I think all of those things need to be carefully considered."
The BBC's Zoe Conway in Washington says the comments go against what in some Washington circles is seen as the accepted wisdom of using the military to curtail Iran's nuclear ambitions.
Speculation had been rife in the American and Israeli media that a strike could take place, either by the Americans themselves or by Israel.
Mr Panetta said he instead supported the use of "the toughest sanctions - economic, diplomatic pressures - on Iran to change their behaviour".
Asked what the US would do if sanctions did not force Iran to change course, he said the hope was that it would not reach that point.
Iran insists that its nuclear programme is for peaceful purposes, to generate civilian power.
In its latest and toughest report so far on Iran, the International Atomic Energy Agency (IAEA) said on Tuesday that it had information indicating Iran had carried out tests "relevant to the development of a nuclear explosive device".
The IAEA said the research includes computer models that could only be used to develop a nuclear bomb trigger.
Tehran condemned the findings of the IAEA as politically motivated.