US military leaders have given away little of their plans for Baghdad. Joint Chiefs of Staff chairman General Richard Myers said at the weekend he did not like to use the word "siege" and the operation would be different from any operation of its kind seen in past conflicts.

But with so much international opposition to the war by US and British forces, experts said the invaders cannot afford a to copy Russian tactics in Chechnya that saw its capital reduced to rubble.

"It's not easy to win a street-fight without destroying the street, and the US Army is aware of this," said John Ferris, a professor of military history at the University of Calgary in Canada.

"The last thing on Earth they are going to do is to go into Baghdad against what they think is going to be stiff resistance. They're only going to do that if they have no other option."

In a policy document on urban warfare released last September, the Joint Chiefs of Staff highlighted the strategy of the ancient Chinese commander Sun Tszu: "The worst policy of all is to besiege walled cities."

Everything will depend on how many Iraqi forces remain loyal to President Saddam Hussein in and around Baghdad, said Ferris.

"If he is able to call on the equivalent of three or four divisions of Republican Guard plus whatever loyal civilian auxiliairies he has, I find it hard to believe that the Americans are going to be able to avoid inflicting massive civilian casualities and massive damage on the city of Bagdhad," he said.

The main problem for US and British forces who have been harassed in southern Iraq has come from paramilitaries and militias, which indicates that the best-equipped forces are being held back for the fight for Baghdad.

Dean Nowowiejski, a former army officer and teacher at the West Point army academy, now an analyst at the Brookings Institution, said US forces have to expect that Saddam will surround himself with his most loyal forces.

"Then it becomes problematic. And you've come down to the very basic question about how much death and destruction has to be inflicted before the regime collapses."

Ferris said the British-led operation to take the southern city of Basra could become a test for the forces that take on Baghdad.

"The British are trying to recalibrate the tactics they used in Belfast to Basra. If it works, and judging purely by television reports so far it seems to have done well in this test-case, they will then believe they have learned a technique that could be feasible for Baghdad," said the military historian.

But after facing stiffer resistance than expected in southern Iraq and with little concrete sign that Saddam's regime is collapsing, the US government has started preparing its people for a long war.

Defense Secretary Donald Rumsfeld and General Myers have said that taking Baghdad is the main prize, but not an easy one. It will probably require a heavy air bombardment of the Republican Guard before the battle can be started.

Clifford Beal, editor of the respected British publication, Jane's Defence Weekly, said rising Spring temperatures in the Gulf region made timing of crucial importance.

"The allies are fighting with kid gloves on, but it'll be difficult to keep this clinical if urban warfare ensues," he said.

"Urban warfare takes longer. It can bog down large numbers of troops. This war is being fought on a clock. And the longer it goes on, the more carnage is seen, the more difficult it is for the Bush administration to continue."

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