Jihadists from the Islamic State group broke into Iraq's massive Baiji oil refinery, seized some of its facilities and are hiding among fuel storage tanks, officials said Wednesday.

IS has attempted repeatedly over the past 10 months to capture the refinery -- the country's largest -- but security forces have managed to fight them off.

The group "now controls the oil (training) institute and the products and shipping department and some of the roads in the refinery," a senior official in the state company responsible for northern refineries told AFP.

"The force stationed at the refinery is fighting fiercely," the official said on condition of anonymity, adding that an operation to clear the facility will be launched in the coming hours.

But militants are hiding among full fuel tanks, and there are fears they will torch them if there is an assault.

An army major general said IS fighters had attacked from two sides and were able to penetrate several hundred metres (yards) into the facility.

"Security forces defending the refinery... continue to besiege (fighters) hiding in some limited locations" and among the storage tanks, the officer said.

The militants are being tracked electronically and visually, he added.

The oil ministry did not respond to repeated requests for comment on the situation.

The US-led coalition providing support to Iraqi forces said it conducted 16 air strikes in the Baiji area from Monday to early Wednesday.

"Coalition airstrikes support (Iraqi) efforts by destroying (IS) buildings, excavators, tactical units, vehicles and weapons," a statement said.

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

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

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

But the jihadists have since wrested the town back.