Ten people were killed in the attack on Thursday when a man blew himself up among a crowd of cadets arriving at the recruitment centre, according to Abdul Karim Khalaf, an interior ministry operations chief.
Some reports said 10 people were killed on the spot and four more died later in hospital.
At least two of the dead are believed to be policemen.
The area around the centre, close to Palestine Street in the Rusafa district of the Iraqi capital, had been placed under a traffic ban after the centre had come under attack three times in the past.
|Gates, left, is under pressure to bring|
home US troops in Iraq [EPA]
Amid the rising violence in Iraq, Gates is under pressure at home to bring the 129,000 US troops back.
As domestic support for George Bush, the US president, and the US-led invasion of Iraq diminishes, Gates is in Iraq for consultation with Iraqi leaders.
On Thursday, Gates had breakfast with US troops.
No soldier present said US forces should be brought home, and none said current troop levels were adequate, as some commanders have argued.
Jason Glenn, a specialist with the US forces, said: "Sir, I think we need to just keep doing what we're doing. I really think we need more troops here ... long enough to where we can get the Iraqi army trained up."
"However you characterise it, it's not good enough"
US defence secretary
Gates has given little indication of the strategies he will recommend to the president when he returns from Iraq, but he has echoed comments from Bush this week that America is "not winning" in Iraq.
"However you characterise it, it's not good enough," Gates told the soldiers about America's progress in the war.
After a defeat in US mid-term elections in November, Bush is expected to announce a new strategy on Iraq early next year.
Nearly 3,000 US soldiers and tens of thousands of Iraqis have been killed since the invasion began in 2003.