US troops reported Thursday pushing to within 15 kilometers (nine miles) south of downtown Baghdad a day after claiming the virtual destruction of at least two crack divisions of the Republican Guard.

But commanders spoke of no major tank battles; nor was there any word on significant Iraqi surrenders or losses other than 500 reported dead at a bridge 30 kilometers (20 miles) southwest of Baghdad.

If the US Central Command (Centcom) has rolled out videos of some of their air strikes every day since the war began two weeks ago, they have produced none of any major hits on the Republican Guard.

So what happened to the tens of thousands of Republican Guard, the hardiest and most loyal of Saddam's fighters equipped with Baghdad's most advanced equipment such as T-72 tanks?

US officials say they just don't know.

"Time will have to tell as to exactly the level of damage that has been inflicted," said Brigadier General Vincent Brooks, deputy head of operations for the US Central Command.

Guard units have taken hits and some have pulled out of their positions and tried to maneuver elsewhere, Brooks told reporters at Centcom's forward command post in Qatar.

But he added: "We know there are a number of forces on the battlefield that have not been engaged in battle. We don't want to be overconfident with what we're seeing. There will still, we believe, be fighting ahead."

Analysts said the quick American advance could mean the Iraqis were drawing them in to the capital for the house-to-house fighting US officials have been determined to avoid.

The New York Times, citing a senior US officer in Washington, said "the enemy is taking what forces he can muster and is ordering them back into the city ... for a last stand."

Other senior brass were sceptical. They said small pockets of troops could make their way back into Baghdad but it would be difficult to infiltrate an entire division of the Republican Guard.

US officials were brimming with confidence Wednesday after reporting significant progress by US army troops toward Baghdad from the southwest and by US marines from the southeast.

They said the Guard's armored Medina division around the city of Karbala and the Baghdad division around Kut to the east were "no longer credible forces" but were reinforced with regular Iraqi army troops.

The Americans positioned themselves at a key intersection and claimed control of all access to the southern approaches of Baghdad, possibly settling in for a siege. They were clearly moving cautiously.

"As to what is inside of Baghdad -- we'll see," Brooks said. "Has the regime extended all of its capability in other areas? Did they use too much of what they had against us? One would have to speculate on that."

The wild card was whether Saddam's forces could be setting a trap for the Americans that they would close with chemical weapons inside a "red line" drawn at some point inside the capital.

Major General Stanley McChrystal, vice director of operations of the Joint Staff, said that as US forces move closer to Baghdad and Saddam's home base of Tikrit to the north, "the likelihood of them using those weapons goes up."

"And so, the posture of our force is prepared for that," he said.

SPACE.WIRE