The 20,000-strong Third Infantry Division, the heavy armored force spearheading the invasion, has concentrated near the Euphrates valley town of Najaf, 150 kilometers (95 miles) south of Baghdad, where many of its troops have been waiting for a week.

"We're finalizing plans for a continued move toward Baghdad," Major John Altman, intelligence officer for the division's First Brigade, told AFP, adding that he expected it to begin "within a week".

Altman declined to go into the specifics of the plan, such as the routes US troops would take.

But coalition warplanes were already softening up Iraqi positions in preparation for the long-awaited offensive even as ground forces tightened up security procedures after a suicide bombing against US troops on Saturday, he said.

Four US soldiers were killed in the kamikaze attack on a desert roadblock, the first of its kind since the invasion began 10 days ago.

The Medina and Hamurabi divisions of the elite Republican Guard, which guard the southern approaches to the capital, have been a particular target of coalition warplanes and artillery, Altman said.

In another "shaping up" operation, coalition air power blew up an oil storage depot near Karbala, north of Najaf, that served to refuel Iraqi forces.

Reconnaissance teams were also conducting probes deep into Iraqi-held territory to determine where Iraqi troops were massing and help US forces select targets, he said.

The Washington Post reported Saturday that covert operations teams were also infiltrating Iraqi cities in a bid to kill Saddam's top aides and commanders.

CIA and other special forces personnel had orders to target ruling Baath Party leaders, Special Republican Guard commanders and other members of Saddam's inner circle, US and foreign officials told the Post.

The paper said the teams were made up of skilled snipers and demolition experts who had killed "more than a handful" of people in the past week.

The fighting around Najaf has been some of the toughest of the war. Around 1,000 Iraqi fighters were killed in the initial clashes up to Wednesday, while another 500 were killed and 600 taken prisoner up to Saturday, Altman said.

US troops have set up a prisoner of war camp in an abandoned building in the desert outside Najaf to accommodate the captives.

Altman said most of the Iraqi dead were members of the Saddam Fedayeen or other irregular militias connected to the Baath party who had launched small-scale attacks against US forces in groups of seven or eight.

The number of Iraqi fighters left inside Najaf had "greatly diminished", even compared with the 1,000 estimated to remain holed up there late last week.

A reported move south from Karbala by thousands of Republican Guard troops had not materialized, Altman said. "They can't further reinforce failure," he surmized.

The hi-tech firepower which US troops have at their disposal can swiftly deal with any Iraqi armor which reveals itself in the open, commanders say.

But although the Third Infantry Division has suffered only five combat deaths -- including the four from the suicide bombing -- it has been caught off guard by the ferocity of Iraqi resistance.

Other factors delaying the march north by "days more than we anticipated" have been a 48-hour sandstorm, supply bottlenecks, and the importance of "maintaining flexibility for eastward movement across the Euphrates," Altman said.

The delays were in sharp contrast to the rapid three-day march US troops had made to Najaf from Kuwait, a distance of more than 400 kilometersmiles).

WAR.WIRE