One suspected member of the paramilitary group died and two others were wounded in the Thursday incident, The Washington Post said.
"Hostile acts will be engaged," Gen. Daniel Bolger, the U.S. commander who overseas Baghdad, said in a statement. "While we value our Sons of Iraq brothers, these men had broken faith with their fellow Sons of Iraq, the Iraqi people and us."
A U.S. military official said an air weapons team spotted four men placing a roadside bomb near Taji "near a critical road juncture" in a rural area close to a U.S. military base, and where several attacks were carried out in recent months.
The U.S. military recently turned over to the Iraqi government control of the Sons of Iraq, many of whom are former insurgents who fought with coalition forces to subdue other insurgents.
In recent weeks, several Sons of Iraq groups have dissolved and some members rejoined militant groups, saying the Iraqi government failed to pay them in a timely manner and was slow to admit them into police academies.