The bomb ripped through a crowd of people waiting to apply for jobs at the offices in Kirkuk, 250 km north of Baghdad. Guerrillas have repeatedly targeted Iraqis queuing up to join the security forces.
Body parts shoes and debris littered the dirt road outside the headquarters. Fire fighters doused flames from a mangled car, and ambulances ferried the wounded to hospital.
Statements purportedly from the Tawhid wal Jihad group of Abu Mussab al-Zarqawi has claimed responsibility for the Baghdad suicide bombings on Friday and Saturday, an website said.
"Two lions of Tawhid wal Jihad, members of the martyrs brigade, today carried out an attack on a group of American soldiers," said one of the statements, whose authenticity was impossible to verify.
A US air strike killed four people and left five wounded in Fallujah late Saturday, as the US military kept up its campaign of nightly attacks in the Iraqi rebel stronghold, a local hospital said.
"Four people were killed and five wounded," said Dr Ali Hiad Mashadani at Fallujah General hospital.
There was no immediate confirmation of the raid from the US military.
In the city of Ramadi, police said they had found the body of the deputy provincial governor. He had been shot dead.
Mohammed al-Zibari, a senior official in the state-run North Oil Company, survived an assassination attempt on Saturday when gunmen attacked his convoy in the northern city of Mosul with automatic gunfire and rocket-propelled grenades, police said. Four of his bodyguards were killed.
On Saturday afternoon a suspected suicide car bomb targeted a US military convoy on the road to Baghdad's international airport. The US military said three soldiers were wounded in the blast, which shattered windows of houses in the area and sent thick black smoke spiralling into the sky.
Moreover, A car bomb attack in Baghdad killed two American soldiers and wounded eight, the US military said in a statement.
The military said the soldiers were on their way to the site of a previous car bomb explosion when they were attacked. Three US vehicles were destroyed in the blast.
Meanwhile, insurgents threatened on Saturday to behead two Americans and a Briton captured in Baghdad.
In Internet video footage the three hostages were shown kneeling blindfolded on the ground, with a hooded gunman aiming his weapon at the head of one of the captives.
The three hostages threatened with death-Americans Eugene Armstrong and Jack Hensley and Briton Kenneth Bigley-were seized from a house in Baghdad by a group of armed men on Thursday. On the Internet footage they gave their names and said they were employed furnishing a base at Taji, north of Baghdad.
The gunman said the Tawhid wal Jihad group led by al-Qaeda militant Abu Mussab al-Zarqawi would kill the men unless female Iraqi prisoners were freed from two Iraqi jails within 48 hours.
"Tawhid sets a 48-hour deadline for the release of all our Muslim sisters in Abu Ghraib and Umm Qasr prisons or else, by God, these three hostages will have their throats slit to set an example," the militant said.
The US military said no women were held at either jail.
Washington regards Zarqawi as its top foe in Iraq and says he has links to al-Qaeda. It has offered $ 25 million for information leading to his death or capture.