US military spokesman Brian Sharkey said the attack occurred at around 06.30 GMT in al-Sulaykh area of Baghdad, on Monday morning.
He added that it was carried out with an improvised explosive device and small arms fire.
The attack takes to 38 the number of US troops killed in action since George W Bush announced an end to major combat operations on 1 May. Another 55 have died in so-called non-hostile incidents.
According to the Pentagon, 152 US soldiers have now been killed in combat during the entire Iraq campaign.
Figures for the number of Iraqis working with the US-led occupation forces who have been killed are not available.
However, anecdotal evidence from families suggests that several may have been targeted and killed for co-operating with the occupiers.
In the worst incident targeting Iraqi nationals, seven police recruits died in a bomb attack in the town of Ramadi last month.
Residents said threats against Iraqis co-operating with US forces in the town had preceded the attack.
Abizaid: Attacks are coming from
the "Sunni triangle"
The latest killing comes after the publication of a Pentagon report on Monday on the guerrilla-type attacks which kill US soldiers almost daily.
General John Abizaid, chief of US Central Command, said in the report the assailants are mostly from the so-called "Sunni triangle" of central Iraq.
Their numbers - "probably several thousand strong" - include mid-level leaders of Saddam Hussein's Baath Party, former intelligence officials, Fedayeen fighters, and Islamic activists from Syria and elsewhere.
Their tactics have become more cautious - they often attack from hundreds of metres away under cover of night, then melt away into the darkness.
Operating in groups of up to 50 men, they communicate with a series of whistles, and track US troops with coloured flares, the report said.
However, Iraqi groups have denied that the attacks are being carried out only by Saddam loyalists from Sunni areas.
They say the attacks are part of a widespread campaign by Iraqi resistance groups to get rid of the American occupiers.
US forces accused of stealing
Meanwhile, a key Iraqi Shia group has accused US forces of staging two raids on its Baghdad headquarters during which they destroyed and confiscated property and equipment.
Al-Adala newspaper, mouthpiece of the Supreme Assembly for the Islamic Revolution in Iraq (SAIRI), said 18 US soldiers entered their headquarters on Saturday afternoon.
They broke down locked doors, tore up armchairs and emptied drawers onto the floor, while pretending to look for documents, Al-Adala said.
The paper also accused the troops of "stealing five computers" and robbing several people, including one visitor of 20,000 dollars.
The report could not be immediately confirmed by US Central Command.