The attack took place in central Baghdad on a busy thoroughfare near the oil ministry, and takes to 50 the number of US troops killed in action since President George W. Bush declared an end to major operations on 1 May.
At least three other soldiers were hurt in the attack, an unidentified US military official told Agence France Presse.
Earlier, rocket-propelled grenades were fired at a tank guarding a hospital in Ba'aquba, 50 km north of Baghdad, injuring nobody.
Jihad against Bush
In a further sign that resistance in Iraq is set to grow, a hitherto unknown group of Iraqis warned in a video tape they would fight a "holy war" against the U.S. President and his administration.
“Bush, Rumsfeld and decision makers in the 'black house' and in the Pentagon - we will shake the ground under your feet and we will send a fire upon you which only God can prevent,” said a masked man on a tape aired by Dubai-based Al-Arabiya.
"America you have declared war on God's soldiers...you won't have security or peace of mind as long as you are an infidel and fighting a war against Islam and Muslims," the masked man on the tape said.
Attacks on occupation soldiers are becoming more frequent as frustration among the Iraqi population grows.
Rolling blackouts and water shortages continue in the country some three months after the overthrow of Saddam Hussein.
US commander in Iraq, Lieutenant General Ricardo Sanchez, yesterday said Iraq was becoming a "terrorist magnet" for foreign fighters.
“In the beginning all the Iraqi people welcomed the Americans," said one middle-aged man, who said only that his name was Mohammed.
"Now the Americans have built a wall between themselves and the Iraqis,” he added
In recent weeks, many groups have claimed responsibility for attacks on occupying US forces.
Search for Saddam
Meanwhile, the search for Saddam Hussein hotted up. US soldiers scoured the deposed dictator's home region of Tikrit.
“We're close to catching Saddam. A lot of people are stepping forward with information. He's running out of places to hide," US army spokeswoman Sergeant Amy Abbot told Reuters.
US Deputy Defence Secretary Paul Wolfowitz, on Sunday accused Al Arabiya and Al Jazeera channels of biased reporting from Iraq, adding that Washington was talking to unnamed governments to try to get more "balanced" coverage.
As many as 60 soldiers have died from non-combat related incidents. Some of these reportedly committed suicides.