Three more were wounded in the explosion near Iskandariya, about 50km south of Baghdad at around 17:00 GMT on Tuesday evening.
Three other US soldiers were also reported killed in a separate roadside explosion in the town of Khaldiya, west of the Iraqi capital.
Aljazeera's correspondent said the soldiers were killed when the vehicle in which they were travelling was struck by an explosive device. At least one US soldier was injured.
Two other vehicles were damaged in the explosion.
String of arrests
The correspondent also said that US forces arrested more than 10 people from around the site of the explosion.
"The last report I have is that three soldiers were killed and one wounded," a US spokesman told Reuters.
The spokesman said a roadside bomb had exploded in the town of Khaldiya at around 10:00 GMT. There are regular attacks on US forces in the Khaldiya region, where resistance to the US occupation is strong.
AFP quoted a witness, resident Ali Makhlef Muhammad, as saying three US soldiers were hurt in the explosion, before American troops shot dead one Iraqi and wounded another.
CNN staff killed
Two CNN employees were killed and another lightly wounded in an ambush in Iraq, the international television news organization said on Tuesday.
Duraid Isa Muhammad, a translator and producer, and Yasir Khatab, a driver, were shot dead as they were returning to Baghdad in a two-car convoy, Atlanta-based CNN said in a Baghdad-datelined release.
CNN cameraman Scott McWhinnie was grazed in the head by a bullet.
Meanwhile, two policemen were killed and another two wounded in an armed attack on one of the checkpoints at the entrance to the al-Faris compound near Amiriyat al-Falluja, west of Baghdad on Monday.
Two Iraqi policemen were killed
in an attack on a checkpoint
"Witnesses said unidentified assailants opened fire at the policemen who were at the checkpoint and then fled", our correspondent said. A search campaign is underway for the attackers.
Iraqis dead in raid
In Tikrit, three armed Iraqis were killed early on Tuesday in raids by the US military in the main oil refinery town of Baiji, a US spokesman said.
"Three armed attackers were killed when they confronted US soldiers raiding four locations simultaneously in Baiji," the distribution hub for production from Iraq's northern oilfields, said Master Sergeant Robert Cargie.
"Soldiers were attempting to capture individuals who were suspected members of Muhammad's Army, an anti-coalition cell operating in the area," said Cargie, a spokesman for the 4th Infantry Division which patrols the area.
Three Iraqis were killed in raids
across Baiji, a refinery town
"The soldiers captured three of the targeted individuals and two others. One of the killed was also targeted."
Cargie recalled that Muhammad's Army had claimed responsibility for at least two attacks on US helicopters.
Graffiti hailing the insurgent group's exploits is widespread across the Sunni Muslim belt that extends north and west from the capital.
An unexploded car bomb was discovered close to the occupation forces' headquarters in Baghdad on Tuesday and was successfully disposed off by US soldiers, the US army said.
Reporters on the scene said the bomb was believed to be in a vehicle in a car park used by visitors to the coalition compound in the "Green Zone", one of Saddam Hussein's former palace complexes which has been taken over by the US-led military and civilian administration in Baghdad.
All roads in the area were sealed off while the bomb was being disposed. The car park is routinely used by media attending press briefings in the coalition compound.
On January 18, a car bomb blast at another entrance to the Green Zone killed at least 25 people and wounded several.