Cpl. Craig Stowell, a spokesman at U.S. command, said the blast occurred just after 10:30 p.m. Thursday, Iraq time, in Abu Ghraib in the western part of the capital.
The casualties were on patrol as part of the 16th Military Police Brigade.
Stowell said the type of explosive used in the attack was not yet determined.
Abu Ghraib, site of a notorious Saddam Hussein-era prison being used by the Americans, has seen occasional mortar fire and other attacks in the past.
Thursday's explosion came on the same day that the 30th Heavy Separate Brigade, based out of Clinton, N.C., was deployed to Iraq. It is the first N.C. National Guard combat brigade to deploy in support of Operation Iraqi Freedom.
The deployment also marked the largest call-up of North Carolina National Guard soldiers since World War II.
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