The pipeline with Turkey was shut down on Thursday, said Ahmad Hassan Ghafif, head of security for gas and oil pipelines in northern Iraq.
"The Iraq-Turkey oil pipeline targeted this afternoon is about 40-inches (1-m) wide and today's attack can be considered the biggest since sabotages started a year and a half ago," he added.
"The explosion cannot be controlled because the pipeline was working at full power and there are large quantities of oil in it," he added.
Ghafif said that the whole oil-rich city of Kirkuk was engulfed in a thick cloud of black smoke and predicted 48 to 72 hours were needed to extinguish the blaze.
Oil prices crossed the $45 mark
on news of the latest blast
"There are common efforts between the fire department, national guard and Iraqi police who have, as a first step, erected sand barriers to prevent the oil from spreading," he said.
"The people in the nearby villages had to be evacuated for fear of intoxication."
The highway was shut down by occupation and Iraqi forces, because the pipeline is just 40m to 50m from it, Ghafif said.
"The expected losses are counted in millions of dollars because the pipeline was working at full capacity," he added.
Oil from the northern Iraqi oilfields around Kirkuk had been pumping at a rate of 600,000 to 800,000 barrels per day in recent days.
The bomb exploded at 6.30pm (1430 GMT) on a road next to the pipeline near the town of Riyadh, 50km south of Kirkuk, said Major-General Anwar Hamid Amin of the Kirkuk national guard unit.
A second bomb was found 5km away from the first, but it failed to explode, Amin said.
"The expected losses are counted in millions of dollars because the pipeline was working at full capacity"
Ahmad Hassan Ghafif,
Head of security for gas and oil pipelines in northern Iraq
The pipeline was attacked at its southern section, where it heads to the country's largest oil refinery in Baiji, before cutting back north towards neighbouring Turkey.
Repeated sabotage of Iraq's oil industry has cost the country more than a billion dollars in revenue. In particular, exports from the lucrative Kirkuk fields had been brought to a virtual standstill by resistance fighters for well over a year.
A recent string of attacks has also damaged several pipelines in southern Iraq. That area accounts for 90% of exports from the country, which sits atop the world's second-largest oil reserves.
Reacting to the disruption, the price of a barrel of oil soared past the $45 mark again in New York.