"All that was left of the bus were its seats, the officers' Kalashnikovs, and human body parts," he said.

Al-Qaeda blamed

Major-General Torhan Yousuf, the deputy police chief of Kirkuk province, told the AFP news agency that the bomber had approached the bus from behind, before driving alongside and crashing into it causing the explosion.

"At this moment we think the attack bears the hallmark of al-Qaeda," he said.

 
"We estimate the bomb contained 250 to 300 kilogrammes of explosives."

Protection of Iraq's oil industry is crucial as the country derives almost all of its revenue from its vast crude reserves, the world's third largest.

About 31,000 people are deployed in the country's oil protection force which safeguards pipelines and provides security at refineries and gas stations.

No oil infrastructure was damaged in Wednesday's blast.

Disputed city

Arabs, Turkmen and Kurds are competing for power in the oil-rich city of Kirkuk, 290km north of Baghdad.

The majority Kurds want to incorporate the area into their autonomous northern region, while other groups want it to remain under central government control.

In December, a suicide bomber killed 55 people in a restaurant outside the city where Kurdish and Arab leaders were meeting to reconcile political differences.

Wednesday's bombing was the latest in a series of deadly attacks that have taken place despite widespread claims that violence is falling across the country.

A total of 252 Iraqis were killed in violence in March.