BAGHDAD (Reuters) - Sunni militants attacked Iraq's largest oil refinery, located in Baiji in northern Iraq, with machine-gun fire and mortars on Wednesday, Iraqi security sources and refinery employees said.

The attack started at 4am (9am Singapore time) from outside two of the three main entrances to the sprawling facility, the sources said.

One mortar hit a spare-parts warehouse and smoke billowed from the building, the sources said. On Tuesday, foreigners were evacuated from the refinery as security forces braced for an attack on the compound.

The refinery has been under siege since Sunni militants began a major military offence in northern Iraq last week.