US Army troops and Marines were advancing on Baghdad from opposite directions Sunday and planned to have Saddam Hussein's capital encircled by the end of the day, a US officer said.

This comes a day after US forces followed up a dramatic seizure of Baghdad's airport by sending dozens of tanks into the capital in a show of force and a bid to probe Saddam's remaining defenses.

British defence analyst William Hopkinson said the Americans will be "more pressured to make themselves felt in Baghdad" than British forces have in Basra, where the British were advancing Sunday after waiting in a siege almost since the war began on March 20 in an effort to spare civilian casualties.

The Americans have the same concern to keep civilian casualties low, said Hopkinson of the London think tank the Royal Institute of International Affairs.

And he said they may be content with encirclement for 48 hours, as well as using "drones overhead, putting in special forces, trying to get the picture."

But he thought that eventually "there will be pressure in Washington that they've go to do something."

Hopkinson stressed the political need to act in Baghad since it is "the seat of the regime. The whole object of the war is to change the regime."

He also said that the Americans want to avoid any hint of a medieval-type siege of the city of five million, with the civilian suffering that would entail -- suffering that would be a public relations coup for Saddam.

In Washington General Richard Myers, the chairman of the Joint Chiefs of Staff, said last week "this notion of a siege and so forth, I think is not the right mental picture."

The scenario he outlined was one in which US forces would cut off the Iraqi regime inside the city and render it powerless.

Hopkinson said this could take the form of seizing specific neighborhoods, mounting hit-and-run raids or snatch missions to seize Iraqi leaders.

"Even if they don't get a leader, the raid itself puts tension on," Hopkinson said.

The London Sunday Telegrah newspaper's defence correspondent Sean Rayment said the Americans are certain to continue their tactic of "exploitation of success," a maneuver warfare concept that entails a force moving forward whenever and in whatever way it can.

Nicholas Rufford wrote in the Sunday Times that "elements of the 101st Airborne Division and a brigade of the 82nd Airborne have been establishing a forward marshaling area in the western desert."

These rapid intervention units "could enter the city down the main highway that leads from Jordan into the heart of Baghdad," Rufford said.

He said: "large stadiums and parks could become landing areas where troops and equipment are flown in by helicopter."

Rayment said that in any case "securing the city will take several days and thousands of trops, even if the Iraqis surrender en masse."

Analysts agreed that the great unknown is just how much fight the Iraqis have left, with the two possibilities being that either the elite Republican Guards have retreated ahead of the American advance in order to reinforce Baghdad or that these troops have been effectively defeated, leaving Saddam's forces disorganized and weak.

The Iraqis "don't seem able to get their act together in terms of coordination," Hopkinson said.

He said command was "devolving to much lower levels" and that this did not say much for the ability of Saddam's military to put up a fight.

Rayment said the questions were "where are the so-called elite Republican Guards and Special Republic Guard units and what are their intentions" and "will the regime, now that its back is firmly against the wall, use chemical weapons."

Also, a key factor is whether the population will rise up in the capital, something which has so far failed to take on regime-toppling proportions in Basra, Rayment said.

As for Basra, the British have taken time there since "once Baghdad falls, Basra will too, with only a few hardliners being willing to fight," according to Hopkinson.

SPACE.WIRE