"The blood is on the hands of the regime. If there's a question of morality, it really should go back to the regime," Brig. Gen. Vincent Brooks, a Central Command spokesman, said.
The United States has blamed Saddam Hussein's government for all civilian deaths in the conflict so far, saying that the regime places civilians at risk by locating military materiel in civilian areas, and dressing fighters in civilian clothes.
In the incident, which took place near the central Iraqi city of Najaf, soldiers motioned for an approaching van to stop, but the driver ignored them, coalition officials said.
The soldiers involved were from the same unit that lost four soldiers at a checkpoint near Najaf on Saturday when an Iraqi soldier dressed as a civilian detonated a car bomb.
The Toyota van approached the checkpoint but did not stop, according to the Central Command statement.
It said soldiers motioned for the driver to stop but were ignored. The soldiers then fired warning shots, which also were ignored. They then shot into the vehicle's engine, but the van continued moving toward the checkpoint, according to the statement. They then fired into the passenger compartment as a last resort, according to the statement.
Of the 13 passengers in the van, seven were killed — all women and children.
A Washington Post reporter, embedded with the 3rd Infantry Division, reported 10 of the 15 civilians in the Toyota four-wheel-drive vehicle were killed, including five children who appeared to be under age five. Two others were wounded, one of whom was not expected to live.
The Washington Post offered an account of the tragedy that suggested no warning shots were fired. The newspaper said Army Capt. Ronny Johnson urgently called for warning shots to be fired as the van approached the checkpoint, but that there was no apparent response from his troops.
Finally, Johnson ordered his troops to open fire on the vehicle, which proved to be packed with women and children.
The Post said that after viewing the carnage through binoculars, Johnson said to his platoon leader: "You just [expletive] killed a family because you didn't fire a warning shot soon enough!"
"It was the most horrible thing I've ever seen, and I hope I never see it again," Sgt. Mario Manzano, 26, an Army medic later told the Post. Manzano said one of the women sat in the van holding the mangled bodies of two of her children. "She didn't want to get out of the car," he told the newspaper.
Central Command said initial reports indicated the soldiers followed the rules of engagement to protect themselves. "In light of recent terrorist attacks by the Iraqi regime, the solders exercised considerable restraint to avoid the unnecessary loss of life," the statement said.
In a similar incident, another Iraqi was killed at a checkpoint near the south-central town of Shatra, according to U.S. Navy Capt. Frank Thorp. Elsewhere, the Central Command said, an Iraqi prisoner was shot to death after he reached for a Marine's weapon while being questioned.
As the line blurs between Iraqi civilians and combatants, another Washington Post reporter said U.S. soldiers in Iraq have been told to take tougher measures
"Everyone is now seen as a combatant until proven otherwise," a Pentagon official said.
Brooks denied there had been any change in the rules of engagement.
"We always maintain the inherent right of self-defense, said Brooks, a spokesman for U.S. Central Command at Camp As Sayliya, Qatar.
"Our checkpoints have to remain alert and vigilant to any type of threat that would approach that which is being protected and secured. We have not had any change in rules of engagement in recent days. There is increased vigilance because of the tactics that we've seen in the battlefield by the regime and death squads that are out there," he said.