The secondary pipeline about 60km west of Kirkuk, which carries oil to refineries in the city of Baiji, was bombed at around 3am (0000 GMT) on Monday, said police officer Sahim Muhammad.
The attack triggered a huge blaze, but firefighters were kept away because Iraqi fighters had warned of dire consequences if they intervened, he said.
The attack came as flames raged in four oil wells after a string of bombings the previous day.
Attackers detonated four bombs near oil wells in the al-Khabbaza oilfield near Kirkuk on Sunday.
Flames were still burning fiercely on Monday, said a security
officer for oil installations.
"The wells are still on fire, after they were attacked yesterday by a group of 30-40 saboteurs who set up explosives in a number of oil wells to the west of Kirkuk," said Major Hammudi Ali.
"Four wells were set on fire and as far as we know, there are
Iraqi security forces have been
unable to secure the oilfields
some other wells with bombs not exploded yet," he said.
"This is the first time the al-Khabbaza oilfield has been targeted since the invasion."
Oil ministry spokesman Assim Jihad condemned the act of sabotage against Iraq's vital oil industry, which generates more than 90% of the country's gross domestic product.
"Such attacks are inflicting harm on the Iraqi economy, as huge sums of money will be spent on putting out the fire, in addition to the fact that large amounts of crude oil are usually wasted," he said.