At least eight people have been killed in the first attack by a female suicide bomber during Iraq's insurgency.
Iraqi security forces have been constant targets for insurgents
Witnesses and army officers said the attacker pushed her way into a crowd at an army recruitment centre in Talafar, northern Iraq.
An internet message purporting to be from the Islamist group al-Qaeda in Iraq claimed a "blessed sister" mounted the attack on its behalf.
Meanwhile US President Bush warned of a spike in violence in coming weeks.
He said the run-up to next month's referendum on a new Iraqi constitution would be a focus for attacks.
"The terrorists have a history of escalating their attacks before key Iraqi milestones," he told reporters at the White House.
"They can't stand elections; the thought of people voting is anathema to them."
Talafar was the scene of major battles between insurgents and US-Iraqi forces earlier this month.
The US military said more than 500 "terrorists or foreign fighters" were killed or captured in the campaign, after which it said the town had been brought back under control.
One of the first steps by Iraqi authorities was to open a military recruitment centre in a former US base - but it was bombed on its first day of operation.
"A suicide bomber blew herself up in front of the recruitment centre. This centre was supposed to be open today for volunteers," Iraqi General Nejam Abdullah said.
28 September 6 killed at army recruitment centre, Talafar
27 September 10 dead at police recruitment office, Baquba
26 September 7 killed in suicide attack at Baghdad police academy
25 September 9 killed in attack on special forces convoy, Baghdad
The bomber blew up a device hidden beneath her Islamic dress as she stood among job applicants, reports said.
Up to 30 people were injured.
One of those, Jumaa Mohammed, said from his hospital bed: "It was a young woman. She pushed her way through the crowd and then there was an explosion."
A statement apparently from al-Qaeda in Iraq said: "A blessed sister... carried out a heroic attack defending her faith... May God accept our sister among the martyrs."
It did not identify the bomber.
The last time women are known to have been used as suicide bombers in Iraq was during the war, in April 2003, when two women, one of them pregnant, blew up their car at a coalition checkpoint, killing three soldiers.
Wednesday's bomb was the fourth major attack on Iraqi security forces in as many days.
At least 10 people died in an attack on police recruits in Baquba on Tuesday.
A day earlier, a suicide car bomber killed at least seven people and wounded 30 outside the police academy in Baghdad.
On Sunday nine people were killed when a suicide bomber drove into a convoy of Iraqi special forces on patrol in the capital.