Iraq's largest oil refinery, located in the city of Baiji, was shut down on Saturday after militants carried out a bomb attack and set it on fire, the governor of Salahuddin province Ahmed al-Jubouri said.

The militants killed four people and planted bombs at production units for kerosene and benzene at the refinery.

"There is a big fire in the refinery and the refinery has completely stopped," Jubouri told Reuters.

The refinery in Baiji, about 180 km north of Baghdad, was controlled for a long time by al Qaeda militants, who used it to finance the insurgency.

In normal circumstances, the refinery daily produces 11 million litres of gasoline, 7 million litres of benzene and 4.5 million litres of kerosene.