Two Iraqi children have been shot dead by US marines at a checkpoint in Nasiriya.
Nine other Iraqis were injured when the marines shot at a car which failed to stop.
A marine spokesman has said the driver of the vehicle ignored a command to stop and the troops feared they were going to be attacked.
They have said in this case it was "a regrettable mistake".
Although Saddam Hussein's regime is falling, the risk of suicide bombers is still real.
Soldiers are therefore continuing to carefully guard these checkpoints at roadblocks.
Suicide bombers are willing to die for their cause.
In Iraq the bombers have already caused injuries and deaths to Iraqi civilians, coalitions troops as well as themselves.