A security official of Iraq's Northern Oil Company, Juma Ahmad, confirmed on Monday that an explosive device badly damaged the pipe at around 1500 GMT.
He said the attack was carried out on a section between the Kirkuk oilfields to the Dibis pumping installations, 50km further north. Pumping had to be stopped in order to tackle the fire.
Issam Muhammad, another security official for Northern Oil, said Monday's fire had been put out, but the damage would take nearly two weeks to repair.
The inability of US occupation forces to provide security in Iraq has prompted retired Marine General Anthony Zinni to criticise White House policies.
The former chief of Central Command told CBS' 60 Minutes it was time to change course in Iraq.
"The course is headed over Niagara Falls. I think it's time to change course a little bit or at least hold somebody responsible for putting you on this course. Because it's been a failure," he said on Sunday.
But Washington dismissed the criticism, saying President George Bush does not look for advice from Zinni, who left Central Command in 2000.
"The course is headed over Niagara Falls. I think it's time to change course a little bit or at least hold somebody responsible for putting you on this course. Because it's been a failure"
former US marine general
White House spokesman Scott McClellan told reporters: "The president looks to those active commanders who are working to implement our policies and build a safer world to make America more secure."
Before his retirement, the general drew up invasion plans that called for deploying 300,000 troops - more than double the roughly 140,000 now in Iraq.
Zinni's scathing critique of the Pentagon and its handling of the war in Iraq are included in a new book about his career, co-written by Tom Clancy, called Battle Ready.