In one of the bloodiest 24 hours since the war in Iraq ended, a second suicide bomber has killed dozens of Iraqis working for the US-led coalition.
Iraqis seen to be collaborating with the coalition are being targeted
Up to 47 people died in the latest attack - a car bomb detonated outside an army recruiting centre in Baghdad.
On Tuesday, a similar attack at a police recruitment centre south of Baghdad killed at least 50 people.
The US military said the bombs bore the hallmarks of al-Qaeda or its affiliate group in Iraq, Ansar al-Islam.
Col Ralph Baker said Islamic militants wanted to launch "sensational" attacks to coincide with a visit by a United Nations team assessing Iraq's readiness to hold elections.
Another US official, Brig Gen Mark Kimmitt, said the closer the summer handover deadline approached, the more attacks were likely.
The US released the text of a letter on Wednesday said to be written by an Islamic extremist with al-Qaeda links, Abu Musab al-Zarqawi, which calls for terror attacks to ignite a civil war between Iraq's Shia and Sunni Muslim communities.
According to the 11-page letter, the discovery of which was announced on Monday after a raid in Baghdad, the "only solution" to defeating the coalition was to begin a civil war.
Wednesday's explosion occurred at about 0740 local time (0440 GMT).
Col Baker said that a car, loaded with 135-225kg (300-500 pounds) of explosives, was driven up to the recruiting centre by a single male.
"It was a suicide attack aimed strictly at Iraqis," he said.
Eyewitness Mohammad Jassim, who suffered cuts to his head, told Reuters news agency: "I was driving and just 10 metres in front of me, a car was driving slowly, suddenly it exploded... I hit a tree."
RECENT MAJOR BOMB ATTACKS
11 February: Up to 47 dead at Baghdad army recruiting centre
10 February: At least 50 killed in attack on police station in Iskandariya
1 February: At least 100 killed by twin suicide bombings in Kurdish city of Irbil
18 January: 18 killed outside coalition HQ, Baghdad
14 December: Car bomb at police station kills 17 in Khalidiya, west of Baghdad
Ghasan Sameer, 32, an officer in the new Iraqi army, who was hospitalised with broken legs and shrapnel wounds, said: "I saw a white Oldsmobile slowly approaching. It ran over some people and exploded. I was blown up in the air and saw fire and body parts all around me."
A security guard who was searching people going into the army recruitment office when the blast occurred described the scene as "apocalyptic".
In another attack on Wednesday, five Spanish peacekeepers and their Iraqi interpreter were lightly injured when an explosive device was thrown at them in the southern town of Diwaniya. Iraqi police later made two arrests.
The latest attack took place near the Green Zone, the high-security area where the US-led administration has its headquarters.
The BBC's Jonny Dymond, in Baghdad, says it is another blow to the creation of the institutions of the new Iraq.
The new attacks represent a change of tactics by militants
The targets are high-profile and relatively unprotected and the message to Iraqis and the outside world is clear - there will be no smooth passage for Iraq to self-government, he says.
A spokesman for the US-led coalition told the BBC that recruits to the Iraqi police and army were coming forward in droves, despite the attacks.
"We have more Iraqis now in security positions protecting Iraqi people that there are coalition forces in the country," Gareth Bayley told the World Update programme.
But our correspondent notes that this also points to the high rates of unemployment in Iraq.