US Defense Secretary Donald Rumsfeld meanwhile expressed confidence that US public support for the occupation will hold despite the mounting casualties. But he acknowledged that improved security will depend on factors beyond the immediate military situation.

"General (Tommy) Franks will root out the remainder of those people to the extent that it can be done," Rumsfeld said. "I think the American people -- and certainly the president and I -- recognize that will take some time, and we think it's important to be done."

Sixteen US troops have been killed by hostile fire since May 1, when President George Bush declared major combat operations in Iraq to be over.

Most recently, a soldier was killed in a drive-by shooting at a Baghdad filling station. Another was wounded in the attack.

In Congress, Republicans and Democrats raised concern that US forces were overextended.

"Because we have long-term commitments in Europe and Asia and long-term requirements in Iraq and Afghanistan and don't know how long the global war on terror will last -- or for that matter, whether it will ever end -- we face a future security environment loaded with uncertainty," said Representative Duncan Hunter, chairman of the House Armed Services Committee.

Rumsfeld, accompanied by retired lieutenant general Jay Garner, the former US administrator in Iraq, told reporters that better security depends "on a whole host of things."

"It's going to be the flow from Iran and the flow from Syria of people. It's going to be the pace at which we are able to get an Iraqi face on an interim authority, and they begin to see that their future is in that direction. It's going to be a function of how rapidly we are going to be able to increase the police forces," he said.

Major General Ray Odierno, the commander of the US Army's 4th Infantry Division, acknowledged his forces were encountering military activity throughout a zone north of Baghdad where support for the regime was strongest.

"But I really qualify it is as militarily insignificant. They are very small. They are very random. They are very ineffective," he said in a videoconference from Iraq.

The attacks appear to be coming from Baathists and former members of the Iraqi security services, Islamic militant groups and poor Iraqis who were being paid a bounty to kill US troops, Odierno said.

But rather than signaling growing popular opposition, Odierno said the attacks were a reaction to pressure applied by US raids in the region.

"So I think they're desperate. I think they're becoming less and less organized," he said.

The Baathists lacked the leaders, weapons and organization to mount an effective challenge to US forces, he said.

"This is not guerrilla warfare. It is not close to guerrilla warfare, because it's not coordinated, it's not organized and it's not led," he said.

US forces have detained hundreds of suspects in 56 raids in recent days, many of them mid-level senior Baathists and former members of the Iraqi intelligence and special Republican Guards, he said.

Among those taken into custody was Abid Hamid Mahmud, Saddam Hussein's personal secretary and chief bodgyguard, whom some believe was the regime's effective number two.

A defense official, speaking on condition of anonymity, said Mahmud was seized along with others in a raid Monday near Tikrit.

Mahmud's unparalleled access to the Iraqi leader and deep involvement in all security matters put him in a position to know what happened to Saddam and to his alleged weapons of mass destruction, making him the biggest US catch yet.

Rumsfeld refused to comment on the capture.

US forces raided two farm houses near Tikrit on Wednesday, netting 15 to 20 people associated with Saddam's personal guard and seizing 8.5 million dollars in cash, as well as a million dollars in jewelry and gems, 300 to 400 million dinars and uncounted euros and British pounds.

"The more money we seize, the more individuals we take into custody, we continue to really, I think, have an impact on the medium-to-senior-level of the individuals that remain," Odierno said.

"So, I think we are, in fact, having a significant impact on them. And I think that's causing them then to come out and be a little bit more desperate in their attacks on US forces," he said.

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