The blast targeted a recruiting centre in central Baghdad, the officials said.
The attack came a day after gunmen killed 12 soldiers and wounded four more at a military site in Al-Adhim, north of the capital.
Militants opposed to the Iraqi government frequently target security forces with bombings and shootings.
Violence in Iraq has reached a level not seen since 2008, when the country was just emerging from a brutal period of sectarian killings.
It took just five days for this month's death toll to top 246, the figure for all of January last year, according to AFP figures based on security and medical sources.
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