BAGHDAD - Gunmen attacked Iraq's largest oil refinery Saturday, killing a guard and detonating bombs that sparked a fire and forced the facility to shut down, officials said.

A general view of the Baiji oil refinery, located some 180 km (112 miles) north of Baghdad, in this Jan 21, 2009 file photo. Iraq's largest oil refinery was shut down on Feb 26, 2011 after militants carried out a bomb attack and set it on fire, the governor of Salahuddin province said. [Photo/Agencies]

The assailants, carrying pistols fitted with silencers, broke into the Beiji refinery around 3:30 am, attacked the guards and planted bombs near some production units for benzene and kerosene, said the spokesman for Salahuddin province, Mohammed al-Asi.

One guard was killed and another wounded, al-Asi said.

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By midmorning, firefighters were still trying to extinguish the blaze, said Iraqi Oil Ministry spokesman Assem Jihad, adding that an investigation will be launched.

"We hope that work will be resumed in a short period of time," Jihad told The Associated Press, but did not give a date. The Beiji refinery has two sections. The attackers targeted the installation's North Refinery that handles 150,000 barrels a day.

The second section, the Salahuddin Refinery, is under renovation. It used to process 70,000 barrels per day. Iraq's overall refining capacity is currently slightly over 500,000 barrels per day. Its three main oil refineries -- Dora, Shuaiba and Beiji -- process slightly over half of the 700,000 barrels-per-day capacity they had before 2003.

Iraq sits on the world's third-largest known oil reserves with an estimated 115 billion barrels, but its production is far below its potential due to decades of war, UN sanctions, lack of foreign investment and insurgent attacks.

At the height of an insurgency from 2004 to late 2007, the Beiji refinery was under control of Sunni militants who used to siphon off crude and petroleum products to finance their operations.

Beiji is about 155 miles (250 kilometers) north of Baghdad.