USS John C Stennis is being deployed to the Persian Gulf

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.

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.

Two triggers

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.

The Natanz plant is buried under concrete, metal and earth

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.

Deadline

Earlier this month US officers in Iraq said they had evidence Iran was providing weapons to Iraqi Shia militias. However the most senior US military officer later cast doubt on this, saying that they only had proof that weapons "made in Iran" were being used in Iraq.

Gen Peter Pace, chairman of the joint chiefs of staff, said he did not know that the Iranian government "clearly knows or is complicit" in this.

At the time, Iranian President Mahmoud Ahmadinejad said the accusations were "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 the BBC it would backfire badly by probably encouraging the Iranian government to develop a nuclear weapon in the long term.

Last year Iran resumed uranium enrichment - a process that can make fuel for power stations or, if greatly enriched, material for a nuclear bomb.

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.

If it does not, and if the International Atomic Energy Agency confirms this, the resolution says that further economic sanctions will be considered.