Iraqi forces backed by US-led air strikes have cleared the country's largest oil refinery of the Islamic State group, the international coalition helping Baghdad fight the jihadists said on Sunday.

IS has made repeated attempts over the past 10 months to capture Baiji refinery north of Baghdad, most recently seizing parts of the facility and holding out for days.

Iraqi forces "regained full control of the Baiji Oil Refinery after having successfully cleared the massive facility of any remaining (IS) fighters," the US-led coalition said in a statement.

The coalition carried out 47 air strikes in the Baiji area over nine days, and Iraq has deployed reinforcements to the refinery and is fortifying it, the statement said.

But Ahmed al-Krayim, the head of Salaheddin provincial council, said efforts to completely clear the refinery were ongoing.

A police officer said that while the facility was "under the control of our forces", there were still holdout suicide bombers and snipers inside.

The refinery -- some 200 kilometres (120 miles) north of the capital -- once produced some 300,000 barrels of refined products per day, meeting half the country's needs.

That output ended when an IS-led offensive overran large areas north and west of Baghdad last June, cutting the refinery off.

An Iraqi operation backed by coalition strikes eventually broke the siege in October and retook Baiji town, just south of the refinery.

But the jihadists later took control of the town again.