The attack happened shortly after midday as the convoy was leaving a main Green Zone exit near al-Mansur district of western Baghdad. Police said the car blew up as a commando unit was passing, spraying shrapnel across a wide area.
Green zone danger
The sound of the blast echoed across the city.
US and Iraqi forces and officials have frequently been attacked at or near entrances to the Green Zone, a huge walled and tightly protected compound on the west bank of the Tigris river in the centre of the city.
In the three weeks since Iraq's landmark elections, at least 100 members of Iraq's security forces have been killed in car bombings and other attacks, while scores have been wounded.
Fighters have attacked Iraqi police and guards in an effort to set back US attempts to create a viable local security force, and because the Iraqi units are less well defended and less well armed than American soldiers.
Also on Tuesday, a number of US soldiers were injured and their vehicle damaged when a car bomb exploded as a US military patrol passed through the al-Sihha neighbourhood in central Mosul city, Aljazeera learned.