The Iraqi regime, which has banked on urban combat to undo a US-British invasion, now faces a sudden, fast-moving drive by US ground forces at the gates of the capital that has raised the prospect of the destruction of its best troops, the Republican Guard forces.

"This is the key battle, obviously," said Michael Peters, a retired US colonel. "If it goes well and the Guards are routed and not able to fall back into Baghdad and somehow coordinate their defenses, then we probably are much closer to the end than to the beginning."

"If, however, this turns out to be a bloodier fight than we would anticipate, certainly then I would anticipate it might take longer and it might also allow them to regroup and resist out of Baghdad," he said.

US military officials reported dramatic gains on the battlefield overnight with one elite Republican Guard division cut off and destroyed at Al Kut, and two others heavily battered in the fighting around the city of Karbala, 80 kilometers (50 miles) south of Baghdad.

Later, officials said US forces had moved to within 50 kilometersmiles) of the capital.

"For sure, what is happening is that the combination of air power and maneuver on the ground is destroying the Iraqi divisions that are in and around Baghdad and in and around Al Kut," General Peter Pace, vice chairman of the Joint Chiefs of Staff, said in an interview with CNN.

US forces have braced for Iraqi use of chemical weapons as they cross a so-called "red line" around the city, but Pace said that so far there have been no reports of its use.

With US tanks on the outskirts of Baghdad and bombs falling on command bunkers in the city, panic in the ranks is likely to become the regime's most insidious enemy, analysts said.

Peters said "it is going to be very difficult" for the regime to retain control and mount an effective defense as US ground forces close on the center.

"Without that kind of central control, the defense is going to be sporadic and it's going to be individual units who are making decisions of whether to fight, and if so how to fight, which means the end is probably near in terms of organized resistance," he told AFP.

The great unknown is how well the regime's security forces inside the city -- the Special Republican Guards, the Fedayeen and other diehard elements of the Baath Party -- will stand up to the pressure.

"Perhaps they have a plan that they are potentially going to execute, and they are going to try to make the battle for Baghdad itself as bloody as it could possibly be," said Peters.

"Their only hope right now, their only hope all along, is to drag this out, to give more time potentially for the rest of the world, particularly the Arab world, to come to their aid," he said.

With civilian casualties mounting and international opinion on the boil, time is working against the United States and its allies.

But analysts here do not expect a headlong rush into Baghdad if US forces break through the defensive perimeters around Baghdad.

Michael O'Hanlon, a military expert at the Brookings Institution, said he believed US forces would wait until the US Army's 4th Infantry Division is in place before assaulting Baghdad itself. The division has just begun unloading its equipment in Kuwait and will not be ready to go for days.

"There are a million options," he told AFP. "They might want to cut off the movement of other Republic Guard units that might want to retreat into the city. That would be one option. Or they might want to get close enough to just start doing more surveillance of the city and figure out where some of the key command headquarters may be provisionally located."

Peters, a former special forces officer, said he expects US forces to move cautiously on Baghdad, using CIA and covert operators as well as technical intelligence gathering to understand what the targets are.

"I think they will take their time and try to do it again in a way to minimize casualties on all sides," he said.

SPACE.WIRE