American military commanders have defended the right of their soldiers to open fire in self-defence.
US troops fear fresh suicide attacks at checkpoints
They were responding to the deaths of seven Iraqi women and children, who were shot in their vehicle by soldiers at a checkpoint.
Two other Iraqis were wounded when the vehicle they were all travelling in was fired on after it reportedly failed to stop at the road block near the city of Najaf, south of Baghdad.
US commanders said they would investigate the incident, but the first reaction was to back the troops.
The soldiers involved "absolutely did the right thing", General Peter Pace, vice-chairman of the Joint Chiefs of Staff, said.
"Our soldiers on the ground have an absolute right to defend themselves."
And at US Central Command in Qatar, Brigadier General Vincent Brooks said there would be no change in the rules of engagement at checkpoints.
"We're trying to get some separation between a potential threat and the force that is being protected," General Brooks told reporters.
"There will be occasions where civilians will be put in harm's way," he said.
Key military developments:
- Intelligence reports suggest that Iraq's elite Republican Guard has moved to reinforce its two most forward divisions south of the capital Baghdad as they come under more intense bombardment
HAVE YOUR SAY
The strategy, if one can call it that, was flawed from the beginning
- Arabic television channel Al-Jazeera says Iraqis have foiled a "landing attempt" by British forces near the northern city of Mosul. UK officials will neither confirm nor deny the incident
- The US says it has seized an Iraqi general, who has provided information on Iraqi deployments
- UK forces say their positions in southern Iraq have come under attack from short-range missiles - the first time Iraqi missiles have been aimed at targets inside Iraq rather than Kuwait.
There are two different versions of what happened. According to coalition Central Command, the soldiers first fired warning shots, then aimed at the engine, and finally targeted the vehicle itself.
But the Washington Post newspaper quotes a senior officer as telling a subordinate: "You killed a family because you didn't fire a warning shot soon enough."
Challenged about the discrepancy between accounts of the checkpoint shooting, an American military spokesman put it down to what he called "the fog of war". He described it as a tragic incident which he wished had not happened.
The BBC's Peter Hunt says the jittery nature of troops on the ground has been heightened by the deaths of four American soldiers on Saturday in a suicide car bomb attack.
This has been reinforced by a report of US marines killing an Iraqi who drove at speed at their checkpoint outside the southern town of Shatra, north of Nasiriya, on Tuesday.
Taken together, our correspondent says, these incidents might hamper efforts by the US-led troops to build a relationship of trust with Iraqi civilians they encounter, as they could create an impression of a force which shoots first and asks questions later.
Advance on Baghdad
Coalition aircraft began a new round of intense bombing of Republican Guard positions on the southern outskirts of Baghdad on Tuesday.
The ground war has also been moving closer to Baghdad after Monday's street-by-street fighting between US troops and Iraqi units including members of the Republican Guard units at Hindiya, about 80 kilometres (50 miles) south of the capital.
HUMAN COST OF WAR
Iraq: 645 civilian deaths, military deaths unknown
US: 43 dead (including 11 in accidents, 2 under investigation), 17 missing
UK: 26 dead (including 15 in accidents, 5 to 'friendly fire')
*Figures from each government
The US attacks are believed to be aimed at testing the strength of the units dug in on the approaches to Baghdad.
US commanders claim that significant damage has been done, not just to Republican Guard units themselves, but to their logistics and support apparatus which makes them into a unified fighting force.
The Iraqi authorities say 19 people died and more than 100 were injured in the overnight raids.
US Secretary of State Colin Powell heads for Ankara and Brussels on the first foreign trip by a senior US official since the war began.
- The Iraqi authorities say two buses carrying American and European "human shields" have been attacked by a US warplane on the Baghdad-Amman highway
- Saddam Hussein says no members of his close family have fled abroad, after the Pentagon claims there is evidence that relatives of senior Iraqi officials have been trying to leave the country