Wednesday's blast came as Iraqis were burying dozens of victims of two attacks against locals working with the US-led occupation.
The US Army's 1st Armored Division said the bomb exploded at 21:30 (18:30 GMT) as the soldiers were passing by in their vehicles.
The casualties were evacuated to a combat support hospital for treatment where two soldiers later died, it said in a statement.
The attack on the US patrol came hours after a car bomb attack killed 47 people at an army recruitment centre in the capital. A similar attack on a police station south of Baghdad killed 53 people on Tuesday.
Only a few of the bodies from Wednesday's blast had been taken for burial. Doctors said some corpses were difficult to identify due to mutilation or bad burns.
The police and new army are central to Washington's plan to hand over power to Iraqis by 30 June. Most of Wednesday's victims were newly recruited soldiers reporting for duty.
Pattern of attacks
The attacks follow a pattern of targeting Iraqis seen as collaborating with the US occupation. Twin bombings in northern Iraq against two Kurdish parties allied with the United States killed more than 100 people on February 1.
47 people were killed and at least
18 wounded in Wednesday's bomb
At least 372 American soldiers have been killed in combat since US-led forces invaded Iraq to topple Saddam Hussein last March.
The violence rages as a UN team, led by veteran diplomat Lakhdar Brahimi, is in the country discussing the possibility of holding elections ahead of the 30 June handover deadline, as demanded by leaders of the Shia Muslim majority. But US plans are for later elections.
Brahimi is due to leave by Friday at the latest, a senior US-led administration official said. The rest of the UN team has started touring provinces. UN Secretary-General Kofi Annan is expected to make a decision on the elections on 21 February.
Two bomb attacks against the UN presence in Baghdad last year killed dozens and forced the UN to pull out of Iraq.
A small explosion on Thursday blew a small hole in a road and smashed windows in a residential area of Samawa in southern Iraq near where Japanese troops are stationed, but there were no reports of injuries, police said.
There is much concern in Japan about the safety of Japanese military personnel who are being sent to help rebuild Iraq in Japan's riskiest military mission since World War Two.
In the southern city of Diwaniyah on Wednesday, five Spanish soldiers on patrol were wounded when an explosive device was thrown at them, Spain's Defence Ministry said.