A military spokesman confirmed the devastating attack on Thursday, naming the Mahmudiya suburb as the scene of the blast.
The wounded marines were taken to hospital within minutes of the 11:30am (08:30 GMT) bombing.
The spokesman was not prepared to provide more details, but did confirm that two more US soldiers were also killed on Thursday following an attack in an Iraqi town north of Baghdad.
The bomb detonated as US occupation forces were patrolling Baquba, 65km north of the capital.
Witnesses said there were numerous additional American casualties, but could not say how many of the troops were dead or injured.
Some 734 US soldiers have died in Iraq since March 2003, with around 4000 reported injured.
With occupation forces involved in fierce fighting in Falluja and Najaf and suffering repeated attacks in the capital, Washington has rushed more tanks and other armoured vehicles to Iraq.
US congressional committees have also been forced to provide the Pentagon with up to $50 billion in emergency money for military operations in Iraq before the election.
Bush had hoped he would not need more money before the 2 November election, but the surge in resistance against the occupation has forced the administration to reconsider.