A man in an explosives-packed jacket blew himself up at a recruitment centre in Sinjar, a town west of the provincial capital Mosul on the road to Syria, and killed 17 people, officials said.
Interior ministry spokesman Major General Abdul Kareem Khalaf said the bomber targeted a centre that recruited personnel for the police and security services in the area.
"Seventeen people were killed in the attack that targeted the recruitment centre," Khalaf told AFP in Mosul, the capital of Nineveh province.
Hospital sources said about 30 people were wounded.
Sinjar police chief Lieutenant Colonel Awad Al Sourge said 14 of the dead were people who had come to the centre hoping to enlist. The other three victims were two policemen and a civilian.
Police said many people had gathered hoping to hand in papers to join the police and security services when the bomber struck.
A few hours earlier, a suicide bomber drove into a group of police officers and detonated his explosives in Al Gabat, just north of Mosul, police Captain Aziz Imara told AFP.
At least three people, including two policemen, were killed and 12 people were wounded, he said, adding that the blast had also damaged shops and restaurants.
The two bombings shattered a relative calm across Iraq. The US military had said violence across the nation had hit a four-year low last week.
Iraqi security forces backed by the US military have been conducting a major crackdown against jihadists in the Nineveh province since May 14 and had claimed that al-Qaeda in Iraq had been largely banished from the region.
Mosul, Iraq's third largest city, is regarded by the American military as the last urban bastion of Al-Qaeda in the country.
Iraqi security forces had said they detained 1,030 suspects during the crackdown and believed that about 2,000 jihadists may have fled to neighbouring provinces as well as the capital Baghdad.
On Monday, Iraqi troops in Mosul said they arrested six teenage boys suspected of being forcibly trained by Al Qaeda to carry out suicide bombings.