The shooting occurred at a checkpoint manned by soldiers from the US Army's Third Infantry Division at Najaf, 150 kilometers (95 miles) south of Baghdad, on Monday afternoon, US spokesman Navy Lieutenant Commander Charles Owens said.

He said the victims, all women and children, were in a vehicle that failed to stop despite repeated warning shots fired by US troops. Two other people in the vehicle were wounded, while four others escaped unharmed.

The Washington Post reported Tuesday that at least 10 civilians had died in the attack, including five children.

Citing officers on the scene, it said the civilians were killed by the 25mm cannon fire, with one man so badly injured he was not expected to live.

US forces in Iraq have been on a heightened state of alert and are approaching Iraqi civilians with much greater circumspection following a suicide car bombing near Najaf on Saturday that killed four soldiers.

Coalition nerves were further frayed by reports at the weekend that as many as 4,000 militants had poured into Iraq and were prepared to commit similar acts of suicide to defend Iraq against the US-British invasion.

Well before Saturday's car bomb, US officers had been making no secret of their outrage at the guerrilla, or "terrorist", tactics they said had been practiced by Iraqi forces -- firing from civilian vehicles, faking surrenders only to open fire, fighting behind human shields.

Troops on the ground have also been frustrated -- and surprised -- by Iraqi forces they say don civilian clothes and then melt into the general population, making it near impossible to distinguish friend from foe.

Reacting to Monday's shooting, Major General Buford Blount, division commander of Third Infantry Division, said Tuesday his troops were "very concerned about it and very sorry that it happened."

But he stressed that with US soldiers on edge after Saturday's suicide car bomb attack, the unit that opened fire on the minibus had respected its rules of engagement.

A written statement issued by US Central Command here early Tuesday said initial reports of the checkpoint shooting "indicate the soldiers responded in accordance with the rules of engagement to protect themselves".

"In light of recent terrorist attacks by the Iraqi regime, the soldiers exercised considerable restraint to avoid the unnecessary loss of life."

Later Tuesday James Wilkinson, spokesman for the US Central Command, described the shooting as an "unfortunate tragedy" and acknowledged that young US soldiers could well be "a little jumpy" in the aftermath of the suicide car bombing.

"While it is a tragedy and we certainly grieve for the loss of the innocent," he told a BBC interviewer, "it does point out the tactics that this regime is going to use.

"In instances like this, the fault lies with the regime."

Owens said the deaths occurred as US troops opened fire on the passenger compartment as a last resort after a vehicle ignored hand signals and two warning shots and continued moving toward them.

"When soldiers opened the vehicle, they found 13 women and children inside. Seven of the occupants were dead, two were wounded and four were unharmed."

As civilian casualties mount in Baghdad, where US and British forces are carrying out day and night air strikes, the checkpoint shooting was a further blow to a US public relations campaign that seeks to portray coalition troops as liberators and providers rather than invaders and occupiers.

"On the hearts and minds front, the war is still not going particularly well for the coalition," said Alan Dupont of the Strategic and Defense Studies Center at the Australian National University.

"While militarily they appear to be regaining the initiative, they've still got a lot of ground to make up on the public relations psychological warfare front."

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