BAGHDAD, Iraq (AP) _ A major pipeline linking oilfields in northern Iraq to the country's biggest refinery was ablaze Thursday, witnesses said.

Sheets of flame and thick black smoke were shooting from the damaged line next to the desert highway about 30 miles north of Beiji, the site of Iraq's largest oil refinery, witnesses said.

There was no immediate explanation about what had caused the blaze. The location of the fire was about 135 miles north of the capital, in the so-called Sunni Triangle where guerrillas have repeatedly attacked pipelines linking Iraq's oil fields with Turkey.

Sabotage of pipelines and other infrastructure has become a major problem for the U.S.-run coalition and its Iraqi partners as they try to revive the country's giant petroleum industry _ the key to economic recovery.

Iraq has the second largest proven petroleum reserves in OPEC. But many companies are holding back until they see an improvement in security against attacks by militants opposed to American troops and the U.S.-backed Iraqi Governing Council.

The northern pipeline which carries crude oil from Kirkuk to Turkey operated briefly in August before one of several attacks on facilities forced the Iraqis to shut it down for repairs.