The victims were among up to 15 civilians reported to be traveling in a van that failed to stop at a U.S. Army checkpoint near the town of Najaf on Monday afternoon. 

The incident occurred two days after a suicide car bomber killed four 3rd Infantry Division soldiers by detonating his vehicle at a checkpoint in the region.

The bombing resulted in new rules of engagement that authorized soldiers to fire on vehicles that do not stop. 

U.S. Central Command in Qatar said Monday's incident began to unfold about 4:30 p.m. [8:30 a.m. EST], when soldiers from the 3rd Infantry Division at the Route 9 checkpoint near Najaf motioned for the vehicle to stop. The driver ignored repeated warnings. 

The soldiers then fired warning shots into the air and the engine that were ignored. Unable to see inside the van, the soldiers fired into the passenger compartment of the vehicle "as a last resort," a Central Command statement said. 

"In light of recent terrorist attacks by the Iraqi regime, the soldiers exercised considerable restraint to avoid the unnecessary loss of life," said Central Command, which added that the soldiers could not see the van's occupants when they fired. 

A Washington Post reporter embedded with the Army's 3rd Infantry Division reported that the vehicle carried 15 people and that 10 of them -- including five children who appeared to be under 5 -- were killed. The reporter also said a man was critically wounded. 

A top U.S. Marine at Central Command called it a "very tragic incident" and said an investigation has been launched. But the vice chairman of the Joint Chiefs of Staff, Gen. Peter Pace, said the soldiers "did the right thing." 

In a story on the paper's Web site, The Washington Post reporter wrote that the unit's captain ordered the first warning shot. As the four-wheel drive Toyota continued down the road, the captain ordered one round fired into its radiator. 

After that failed to stop the vehicle, according to the Post reporter, the captain yelled "Stop him, Red 1, stop him!" That was followed by about a half-dozen shots from the 25 mm cannon on the platoon's Bradley armored vehicle, the reporter wrote. 

"You just [expletive] killed a family because you didn't fire a warning shot soon enough!" the reporter quoted the captain as yelling at the platoon leader. 

'The most horrible thing I've ever seen'
Commenting on the incident early Tuesday, CENTCOM spokesman James Wilkinson told CNN that the newspaper account does not match the report from field commanders. 

"On the battle field we have this phenomenon called 'fog of war," he said. "We continue to see reports from embedded reporters that have discrepancies from our headquarter's reports. What I can tell you is that right now as we speak, we are working to reconcile this." 

An Army medic was quoted in the Washington Post as saying, "It was the most horrible thing I've ever seen, and I hope I never see it again." 

Central Command said it appeared the proper rules of engagement were followed. 

"They absolutely did the right thing," Pace, a Marine, said Monday in an interview on PBS' "The NewsHour with Jim Lehrer." 

"They tried to warn the vehicle to stop," Pace said. "It did not stop." 

Pace blamed the shooting on the "environment" created by recent incidents in which Iraqi soldiers have dressed as civilians and used women and children as shields. 

"Our soldiers on the ground have an absolute right to defend themselves. They will always, if they can, find a way to stop a vehicle like that without having to actually to fire at it. But in the final analysis, when their lives are threatened, and of course they thought they were, they will shoot." 

In an interview with CNN's Larry King, Col. Tom Bright -- the Marine Corps chief at the U.S. Central Command's joint operations center in Qatar -- said simply, "It was a very tragic incident."