Attackers used a live bomb to kill at least nine Iraqi security force members dismantling an apparent decoy device near the oil-rich city of Kirkuk.
A series of insurgency bombing also rocked Baghdad
The casualties, including at least three wounded, worked for the Northern Oil Company guard service.
Anti-US insurgents also launched a string of attacks in Baghdad, as the second high-level American official visited Baghdad in as many days.
At least four roadside blasts have been reported in the Iraqi capital.
The US state department's second-in-command Robert Zoellick arrived there a day after Defence Secretary Donald Rumsfeld's one-day stopover.
Both visits were kept secret for security reasons until the officials arrived.
Police say the security guards killed in the blast had been cordoning off an area containing a suspected decoy device when a second, live device blew up.
They included Colonel Natham Abdullah who was in charge of protecting the northern oil fields from sabotage, police said.
A large force has been created by Iraq's defence ministry to guard the oil and electricity infrastructure, which has been a frequent target of anti-US insurgents.
In Baghdad one bomb struck an oil tanker in the east of the city, sending thick clouds of black smoke into the air.
Two other bombs went off near US military convoys, one on the road to the airport and another in the Amariya district. No US casualties were reported but at least seven Iraqis were wounded.
In other violence on Wednesday, the US military said it had killed a suspected foreign insurgent in a fierce gun battle near the restive western city of Qaim, near the Syrian border.
Local hospital officials are quoted as saying that at least nine people were killed in clashes in Qaim but they were all believed to have been civilians.