US contingency plans for air strikes on Iran extend beyond nuclearsites and include most of the country's military infrastructure, theBBC has learned.
It is understood that any such attack - if ordered - would targetIranian air bases, naval bases, missile facilities andcommand-and-control centres.
The US insists it is not planning to attack, and is trying to persuade Tehran to stop uranium enrichment.
The UN has urged Iran to stop the programme or face economic sanctions.
But diplomatic sources have told the BBC that as a fallback plan,senior officials at Central Command in Florida have already selectedtheir target sets inside Iran.
That list includes Iran's uranium enrichment plant at Natanz.Facilities at Isfahan, Arak and Bushehr are also on the target list,the sources say.
BBC security correspondent Frank Gardner says the trigger for such anattack reportedly includes any confirmation that Iran was developing anuclear weapon - which it denies.
Alternatively, our correspondent adds, a high-casualty attack on USforces in neighbouring Iraq could also trigger a bombing campaign if itwere traced directly back to Tehran.
Long range B2 stealth bombers would drop so-called "bunker-busting"bombs in an effort to penetrate the Natanz site, which is buried some25m (27 yards) underground.
The BBC's Tehran correspondent Frances Harrison says the news thatthere are now two possible triggers for an attack is a concern toIranians.
Authorities insist there is no cause for alarm but ordinary people are now becoming a little worried, she says.
Earlier this month US officers in Iraq said they had evidence Iran wasproviding weapons to Iraqi Shia militias. However the most senior USmilitary officer later cast doubt on this, saying that they only hadproof that weapons "made in Iran" were being used in Iraq.
Gen Peter Pace, chairman of the joint chiefs of staff, said he did notknow that the Iranian government "clearly knows or is complicit" inthis.
At the time, Iranian President Mahmoud Ahmadinejad said the accusationswere "excuses to prolong the stay" of US forces in Iraq.
Middle East analysts have recently voiced their fears of catastrophic consequences for any such US attack on Iran.
Britain's previous ambassador to Tehran, Sir Richard Dalton, told theBBC it would backfire badly by probably encouraging the Iraniangovernment to develop a nuclear weapon in the long term.
Last year Iran resumed uranium enrichment - a process that can makefuel for power stations or, if greatly enriched, material for a nuclearbomb.
Tehran insists its programme is for civil use only, but Western countries suspect Iran is trying to build nuclear weapons.
The UN Security Council has called on Iran to suspend its enrichment of uranium by 21 February.