The extent of the US advance was symbolized by the all-but-complete seizure of Baghdad airport, with pockets of resistance remaining.

British Defence Secretary Geoff Hoon said this was a "huge psychological blow" to the regime of Iraqi President Saddam Hussein.

He said the people of Baghdad have "been told by elements of the regime, the leadership that there are no coalition forces anywhere near Baghdad. They will be able to see for themselves soon how untrue that is."

It was far from certain however that the toppling of Baghdad would be as easy as the march on Saddam's fortress, where the Iraqi leader hopes to draw US-British forces into urban warfare that will negate their technological superiority and lead to massive Iraqi civilian casualties that will destroy the allies politically.

US troops have "certainly moved quickly and easily to the fringes of Baghdad but that very easy movement raises questions," William Hopkinson of the London think tank, the Royal Institute of International Affairs, told AFP.

He said "one must conclude that a lot of (the elite) Republican Guards (troops), with their tanks, have withdrawn to Baghdad."

And such an ordered withdrawal, if this is the case, is exactly how Saddam should fight against the massively superior firepower of the US-British blitzkrieg, against which his troops are helpless in open battle, Hopkinson said.

Apparently, the Iraqis have "withdrawn in very good order, keeping their valuable assets and not losing anything on the way," Hopkinson said, referring to tanks and even possible chemical and biological weapons.

He called the airport, 18 kilometres (12 miles) from the capital, "an outward and visible" symbolic conquest, "rather than a grand strategic matter."

But Ian Kemp, an expert in urban warfare for the prestigious military magazine Jane's Defense Weekly, said he thought the Republican Guards have lost a signifiant amount of hardware.

"There were considerable casualites," Kemp said, saying the Republican Guards had before the war only 500 tanks, with 100 assigned to the special unit positioned inside Baghdad.

This left 400 tanks "split over a wide area between the north of Baghdad and the south of Baghdad" and that is why "you are certainly not seeing hordes of tanks destroyed," Kemp said.

The US entry into the airport was a "major coup" showing lack of resistance from Iraq's elite troops.

Kemp said the coalition forces would "continue to tighten the noose" around Baghdad.

But beyond that it was impossible to say what exactly would happen next, or how fast it would take place, Kemp said, adding that reports about Saddam fleeing or ready to flee to his northern city of Tikrit or about an injured Saddam not being in control anymore were impossible to confirm.

"It all now depends on the will of the defenders of Baghdad," he said.

He said the Americans would probably do something similar to what the British were doing in the southern city of Basra, waiting outside the town for the population to rise up and in order to avoid the civilian casualties a frontal attack would cause.

"I certainly expect some psychological operations campaign," he said, referring to a range of tactics from distributing propaganda leaflets to carrying out surgical strikes such as "snatches" of Iraqi leaders.

Kemp said Baghdad would undoubtedly fall before Basra since the coalition soldiers estimate the Basra garrison will only give up if their leaders in the capital are taken.

This is why coalition troops would do street fighting if necessary in Baghdad, something for which they are well trained.

"They will start to move in and clear selected areas," Kemp said, but only after Saddam's forces in Baghdad manage to survive what are certain to be precision ground and air strikes and a siege-like cut-off of electricity and water.

WAR.WIRE