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 were in a vehicle that failed to stop despite hand signals and repeated warning shots fired by US troops, who then fired at the passenger compartment as a last resort.

Citing officers on the scene, he said the civilians were killed by 25mm gunfire, with one man -- apparently the driver -- 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 caution following a suicide car bombing near Najaf on Saturday that killed four soldiers.

Well before Saturday's car bomb, US officers had been making no secret of their outrage at the guerrilla 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, 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 the 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."

Central Command spokesman James Wilkinson later 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 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."

US Brigadier General Vincent Brooks told a press briefing Tuesday at Central Command's forward planning base in Qatar, "While we regret the loss of civilian lives ... they remain unavoidable as they have been throughout history."

"Our efforts may result in the loss of civilian lives and they clearly will result in the loss of Iraqi military lives," he also said when asked if the checkpoint shooting would affect the coalition's efforts to win "the hearts and minds" of the people of Iraq.

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.

US newspapers warned of the negative fallout from the killings on public opinion in Iraq as well as the Middle East at large, and said such incidents play into the hands of President Saddam Hussein's regime.

The balancing act for military commanders to minimize civilian casualties while putting down Iraqi resistance is "clearly growing harder," the Washington Post said.

"Even if accidental, such events, like the deaths of civilians in Baghdad attributed to errant US bombs, can incur large political costs both in and outside Iraq," the Post said.

The New York Times said that "billions around the globe are seeing and hearing reports that women and children were gunned down yesterday ...

"This is just what the Iraqi commanders have in mind when they send soldiers disguised as non-combatants to fire on unsuspecting American troops," the paper said.

"If such scenes become routine ... , the political war for Iraq could be lost even before the military one is won," warned the Times.

The International Committee of the Red Cross (ICRC), meanwhile, said it regretted the shooting and hoped US troops made sure that the driver of the minibus, had been adequately warned.

"We hope that they took all the necessary measures to warn the people, to clearly alert them," Antonella Notari, an ICRC spokeswoman in Geneva, told reporters.

"Checkpoints have, of course, the right to try and defend their own security and the security of their personnel so they may search vans, they may stop vehicles, there are certain rules on how you proceed in that," she said.

"I can't comment on what exactly happened on the spot, I can only deeply regret that again they were civilians who were hit," she added.

SPACE.WIRE