Gunmen shot their way into Baiji [Bayji] refinery early on Saturday, killing five members of staff, according to the Washington Post. Guards killed two of the gunmen, but the rest escaped.
The gang set off explosions that forced the facility to shut down for at least two weeks, officials said.
Baiji, situated about 150 miles north of Baghdad, is Iraq's largest oil refinery, accounting for about half of the country's 550,000 barrels of oil a day refining capacity. It produces gasoline for cars and fuel for power plants, and a prolonged shutdown could cause major headaches for the government.
About 45 soldiers have been brought in temporarily to protect the facility.
In the southern town of Samawa, a second refinery was shut down by fire, but officials said initial reports indicated it was started by a technical failure rather than an insurgent attack.
(Sources: Associated Press, Washington Post, Reuters)