The commander of US ground forces, Lieutenant General Ricardo Sanchez, said on Thursday that the blaze broke out 9km north of the town of Baiji. He did not give any other details.
The pipeline is one of several 20-inch lines from the Kirkuk oilfields that feed the main export line to Turkey, said a firefighter at the scene in Baiji.
Bahaa Hasan, firefighting manager for Iraq's Northern Oil Company said it would take a "few days" to repair and would have no impact on the timetable for the restoration of exports from Kirkuk for the first time since the US invasion in March.
Earlier US military spokeswoman Major Josslyn Aberle had told reporters she thought the new blaze was on a line feeding the Baiji refinery.
Bahaa Hasan said it was not clear whether or not the blaze, north of Baiji, was another in the series of resistance attacks that had affected oil sales in US-occupied Iraq.
Iraq's northern Kirkuk exports have remained closed since the invasion. The US military a few days back said it hoped to see the line reopened by about mid-October.
Baghdad has relied on deliveries from its southern fields for vital export revenues to help finance post-war rebuilding efforts.
The northern pipeline runs southeast from the Kirkuk oilfields to the Baiji refinery, before pumping northwest across the Turkish border to the Ceyhan export terminal on the Mediterranean.