The attack, apparently using a cement truck, comes 12 days after a car bomb at the Jordanian Embassy in the city which killed at least 11 people.
The assault on a major UN facility broadens the assault on international representatives in the country, adding to the campaign of guerrilla attacks which have hit US and British troops.
They come alongside attacks by saboteurs on Iraq’s infrastructure and oil industry, which are thought to have cost billions of dollars.
At least 60 Americans have died since President George W Bush declared major fighting over on May 1.
The guerrilla campaign has mainly been waged using hit-and-run shootings by small groups, or roadside bombs detonated by remote control.
On Monday, a US soldier was fatally wounded when an explosive device detonated in Baghdad, and two more were wounded when their convoy was attacked with grenades and small arms fire about eight miles east of Saddam’s home town, Tikrit.
Last Thursday, Captain David Martyn Jones of the 1st Queen’s Lancashire Regiment was killed when the ambulance he was travelling in was targeted near Basra, southern Iraq.
He was the first British soldier to die in combat since June 24, when six Royal Military Police were killed as a crowd surrounded a police station at Al Majar Al-Kabir, near Basra.
They were attempting to defend the police station after a confrontation in which locals complained about heavy-handed police searches.
Saboteurs have been attacking water, petroleum and electrical lines.
Paul Bremer, the US civilian administrator in the country, conceded that the attacks on the infrastructure are hampering US rebuilding efforts.
He has said: “It’s people who do not share the vision of a free Iraq with a vibrant economy the president has set forth and which Iraqis share.
“These are probably people left over from the old regime who are simply fighting a rearguard action.”