The morning attack at the recruitment centre in Riyadh, a mostly-Sunni town in ethnically-mixed Kirkuk province, injured 14 other recruits, according to army Major General Mohammed Khalaf al-Dulaimi.
Dr Mohammed al-Juburi at the main hospital in nearby Hawijah confirmed the toll.
The attack comes amid a surge in violence, Iraq's worst since it emerged from a brutal sectarian war that left tens of thousands dead in 2008, with just weeks to go before parliamentary elections.
More than 2,200 people have been killed so far this year, according to an AFP tally based on reports from security and medical officials.
Analysts and diplomats have urged the authorities to reach out to the Sunni community in a bid to undercut support for militancy, but political leaders have been loath to compromise with the elections looming.