For most experts a bombing strike against a limited number of suspected nuclear sites is the most likely option being considered.

The GlobalSecurity.org consultancy said there are about two dozen suspected nuclear facilities in Iran and that the Bushehr 1,000 megawatt nuclear plant would be a prime target.

Also the suspected nuclear facilities at Natanz and Arak will likely be targets of an air attack by B-2 or F-117 bombers, it said in an analysis on the crisis.

"American air strikes on Iran would vastly exceed the scope of the 1981 Israeli attack on the Osiraq nuclear center in Iraq, and would more resemble the opening days of the 2003 air campaign against Iraq," GlobalSecurity.org said.

But Peter Brookes, an expert on national security and foreign policy at the Heritage Foundation think-tank, said that "flattening Iran's nuclear infrastructure isn't easy or risk-free" because most of the facilities are underground.

"Iran burrowed many sites deep below the soil, making them much tougher targets (it also put some sites near populated areas to make civilian casualities a certainty if attacked)," he said in a report.

Brookes said there were about 20 known nuclear sites across Iran but the final figure could be higher than 70.

The United States could also carry out a "more comprehensive set of strikes" against nuclear and other military targets "that might be used to counterattack against US forces in Iraq", GlobalSecurity.org said.

Another option would be an invasion, but with US forces already stretched in Iraq such an operation appears unlikely.

But US Army Secretary Francis Harvey said last week that the United States could deploy 15 extra brigades, between 45,000 and 75,000 troops, if it faced a new crisis.

Iran's reaction to any kind of military action remains a mystery.

Joseph Cirincione, director for non-proliferation at the Carnegie Endowment for International Peace, said an attack could "rally the Iranian public around an otherwise unpopular government and jeopardize further the US position in Iraq."

"The strike would not, as is often said, delay the Iranian program. It would almost certainly speed it up," he commented.

Cirincione called the Israeli strike in 1981 "a tactical success but a strategic failure" because it accelerated the Iranian nuclear programme.

Brookes also warned of a potentially extreme response by Iran against the United States and Israel.

"The Iranian regime is already up to its neck in the insurgencies in Iraq and Afghanistan. It could certainly increase its financial/material support to the Sunni insurgents, Shia militants, Al-Qaeda and the Taliban to destabilize the new Baghdad and Kabul governments," he said.

"It could also mess with other nations' oil exports -- attacking tankers in the Gulf using mines, subs, patrol boats or anti-ship missiles.

"The mullahs could unleash their terrorist attack dogs Hezbollah, Hamas and Palestinian Islamic Jihad against Israel, killing untold numbers in suicide attacks."

The Atlantic Monthly magazine staged a war game simulation of such an attack in 2004 with security experts.

It concluded that the United States had "no way of predicting the long-term strategic impact of such a strike.

"A strike might delay by three years Iran's attainment of its goal -- but at the cost of further embittering the regime and its people. Iran's intentions when it did get the bomb would be all the more hostile," the Atlantic Monthly said the experts concluded.

Iran insists it is not seeking to build nuclear weapons, but that it has the right to develop atomic energy for peaceful electricity generation.

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