The fire was extinguished and oil had stopped flowing through the pipeline which was reopened recently.
"The Baiji pipeline was disrupted by a fire which was contained as of one hour ago. It took some 24 hours to extinguish it," said Lt-Col William MacDonald, chief public affairs officer of the 4th Infantry Division.
It was not clear how long it would take to repair the pipeline, which had begun moving Iraq's Kirkuk crude for the first time since the war that toppled Saddam Hussein on 9 April.
Eyewitnesses said a fire was blazing on Saturday at a pipeline in the town of Baija, home to Iraq's biggest refinery.
The damage to the Iraq-Turkey pipeline is the latest setback to efforts to rebuild Iraq's oil industry and generate much needed revenues for reconstruction.
Iraq has struggled for months to repair its northern oil export pipeline after a spate of blasts which have stopped exports.
"One section was repaired in 10 hours and we are working on another right now," said. Colonel Robert Nicholson, chief engineer for the 4th Infantry Division based in northern Iraq.
Oil installations have been key
targets for anti-occupation forces
Nicholson said aside from technical problems from the pressure after the pipeline reopened, there was also other
damage over the past few days that may have been from resistance attacks.
"One section was repaired in 10 hours and we are working on another right now," he said. "There were two cases of unexploded bombs that were found near the pipeline."
Iraq also faces the challenge of keeping oil flowing through a pipeline that has suffered from a lack of investment due to 13 years of economic sanctions.
"There is a section with several things wrong. There has been no capital investment for at least 12 years. There were also unwise practices in the haste to get the oil out," Nicholson said.
Iraq had been exporting all of its oil from the south before the northern pipeline reopened. But the southern region is also facing problems. Theft of power lines has halved exports and threatens to stop sales altogether.