Iran: The attack plan
US contingency plans for air strikes on Iran extend beyond nuclear sites and include most of the country's military infrastructure, the BBC has learned.
It is understood that any such attack - if ordered - would target Iranian air bases, naval bases, missile facilities and command-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.Alternatively, our correspondent adds, a high-casualty attack on US forces in neighbouring Iraq could also trigger a bombing campaign if it were traced directly back to Tehran.
But diplomatic sources have told the BBC that as a fallback plan, senior officials at Central Command in Florida have already selected their 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 an attack reportedly includes any confirmation that Iran was developing a nuclear weapon - which it denies.
Long range B2 stealth bombers would drop so-called "bunker-busting" bombs in an effort to penetrate the Natanz site, which is buried some 25m (27 yards) underground.
The BBC's Tehran correspondent Frances Harrison says the news that there are now two possible triggers for an attack is a concern to Iranians.
Authorities insist there is no cause for alarm but ordinary people are now becoming a little worried, she says.