"We're very concerned about it and very sorry that it happened," Major General Buford Blount, division commander of the Third Infantry Division, said.

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

His words echoed an official statement issued by US Central Command, which in part read: "In light of recent terrorist attacks by the Iraqi regime, the soldiers exercised considerable restraint to avoid the unnecessary loss of life."

The shooting occurred Monday at a checkpoint near Najaf, 150 kilometersmiles) south of Baghdad.

The seven victims were in a vehicle peppered with shots by soldiers from Blount's division.

US military officials in Qatar said the vehicle had not stopped at the checkpoint despite the US troops reportedly firing warning shots. Two other people in the vehicle were wounded, while four escaped unharmed.

Blount was at a loss to say why the vehicle did not heed instructions to stop before the checkpoint, and speculated that maybe they "were trying to escape from Karbala and thought this was a regime checkpoint."

He said he had heard that Iraqi death squads had taken over homes in Karbala, which lies north of Najaf, and were forcing families living near a hospital to leave.

He also said that his forces had in one encounter seen Iraqi fighters herding women and children ahead of them as human shields.

In that incident, one woman who broke away was shot in the back by the Iraqis and US troops put up a smokescreen to rescue her and fly her out for medical treatment.

The general said that his men had experienced "lots of contact" in the region and believed that Iraqi militias loyal to President Saddam Hussein were moving their fighters around using buses.

Because of the hostile situation, the soldiers who killed the civilians Monday had merely been enforcing security measures, he said.

"The bus committed a hostile act. It may have been unintentional," he said.

Captain Jack Pritchard, a legal advisor to the troops, said that the rules of engagement obliged troops to use the minimal amount of force necessary to stop a threat and minimise collateral damage but in some instances you can use deadly force.

"After the taxi thing (Saturday's suicide attack) and the bus, we thought: how can you change the rules (of engagement). You can't and not violate the laws of war," said Pritchard.

"When we fight a fair fight and they don't, our soldiers are going to die. We are just going to have to live with that."

Marine Sergeant Christian Driotez said that while some Iraqis had been approaching checkpoints with white flags, others had been pulling up close by and shooting.

"We do everything possible to get a positive ID before we even fire a round off," said Driotez.

"I think we have been as careful as we can be distinguishing between civilians and guerrillas."

Orders were now going out to try to limit repeats of such incidents, particularly by way of putting up more barriers before the checkpoints to force drivers to a complete halt well before the troop positions, Blount said.

Civilians would also be encouraged to stay at home and not move around.

SPACE.WIRE