Sunni militants have taken control of most of Iraq's largest oil refinery, located in Baiji in northern Iraq, an official at the refinery said on Wednesday.

"The militants have managed to break in to the refinery. Now they are in control of the production units, administration building and four watch towers. This is 75 percent of the refinery," an official speaking from inside the refinery said.

He says clashes continue near the main control room with security forces.