"Forty-seven people were killed and more than 50 injured," General Ahmad Kathim, the Iraqi Interior Ministry undersecretary told Aljazeera's correspondent.
He said the driver of an explosive-laden car headed towards a military training centre and detonated the bomb among a group of volunteers for the new Iraqi army.
Body parts were scattered at the explosion site.
The correspondent added heavily deployed US forces and Iraqi policemen had cordoned off the area known as Muthana Airport, a former abandoned military air facility, but recently used by the new Iraqi army. Offices of a Shia Muslim group are situated nearby.
Journalists have been prevented from entering the area.
Aimed at Iraqis
US Colonel Ralph Baker told Reuters: "It was a suicide attack by a single male who was killed."
"It was aimed strictly at Iraqis," he added. The men had reportedly been queuing up to join the Iraqi army.
Occupation troops immediately
sealed off the area
About 50 people were killed in a similar attack on Tuesday against Iraqis outside a police station in the small town of Iskandariya, 40km south of Baghdad.
The US military said Wednesday's attack occurred at around 7:40 am (0440 GMT) when a white car drove into the new Iraqi army facility and exploded.
The attacks followed a pattern of targeting Iraqis seen as collaborating with the US occupation.
Twin bombings in northern Iraq against two Kurdish parties allied with the United States killed more than 100 people on 1 February.
Iraqis seen to be fraternising with
US troops are targeted
Tuesday's bomb exploded among civilians who had queued outside a police station to apply for jobs.
At least 75 people were wounded and the police station and an adjacent court were badly damaged.
One Iraqi man at the al-Iskandariya blast scene told Aljazeera's correspondent: "The Americans are behind these explosions in order to give the UN the impression that Iraq is insecure and incapable of holding early direct elections."
Iraqi officials say 300 policemen - who have been regular targets of bombings - have been killed by resistance fighters.
"It's impossible to defend in every location against every conceivable kind of attack at every time of the day or night," US Defence Secretary Ronald Rumsfeld told reporters in Washington after Tuesday's blast.
Rumsfeld said not every location
could be protected against attacks
At the same briefing, Air Force General Richard Myers, chairman of the military's Joint Chiefs of Staff, said he was optimistic about security despite the attack.
"We continue to be optimistic about the situation on the ground in Iraq." There has been "a lot of success," Myers said, in bringing stability and security to Iraq before the 30 June target date for handing over power to an Iraqi government.
Rumsfeld said between 150,000 and 210,000 Iraqis were working in the security forces, adding they were getting better at it all the time.
"That does not mean that there will not be people that are killed. I mean, look at any city on the face of the earth. Everyone's against homicide. And yet in every... major city on the face of the earth, homicides occur every week. Hundreds occur every year in every city.
"Now, why if we have all those policemen, why if we have everyone against homicides, do they still occur? The answer is because human beings are human beings."