Iraq forces are battling Sunni militants for control of the country’s biggest oil refinery.

A military spokesman said troops had repelled repeated attacks by the militants, led by the jihadist Islamic State in Iraq and the Levant (ISIS).

Forty assailants were killed overnight and on Wednesday, he added, denying the facility was close to being overrun.

But some workers said the militants had seized most of it and that troops were concentrated around the control room.

It comes as President Barak Obama considers a formal request from Iraq’s government for air strikes against the militants, who have seized several northern cities and towns since last week.

ISIS-led militants launched an assault on the Baiji refinery, about 130 miles north of the capital Baghdad, early on Wednesday.

Workers trapped inside said the attackers had breached the perimeter and destroyed one of its fuel tanks. Some said they had taken control of most of the facility. Video footage showed smoke billowing from the refinery and the black flag used by ISIS flying from a building.

However, officials insisted that troops had resisted the advance.

Nearly all the 15,000 workers at the refinery and 100 foreign experts left on Tuesday when the plant was shut down in anticipation of the attack.

On Thursday morning, the remaining 250 to 300 workers were evacuated under an agreement brokered by local tribal leaders, one of the workers told the Reuters news agency.

The battle over Baiji, which supplies much of the country’s domestic fuel, has sparked fears that Iraqis will soon experience long lines at petrol pumps and electricity shortages.

On Wednesday there were fierce clashes in the Baquba, about 37 miles north of Baghdad, as jihadists again tried to enter the city centre and seize its prison.

Shia militiamen have been sent to assist in the defense of the capital of Diyala province, which has effectively become a frontline, and the nearby city of Samarra, site of a major Shia shrine.

PM Nouri al-Maliki has tried to assure Iraqis that the momentum is shifting.

The Iraqi government is awaiting President Barack Obama’s decision on air strikes.

On Wednesday, the chairman of the US Joint Chiefs of Staff, Gen. Martin Dempsey, warned that the US military still lacked sufficient intelligence to take action. He told a congressional hearing that pilots would have difficulty knowing who they were attacking from the air.

Asked if the intervention might come too late, Gen Dempsey said: “There is very little that could have been done to overcome the degree to which the government of Iraq has failed its people. That is what has caused this problem.”

ISIS has exploited deep resentment among Iraq’s minority Sunni Arabs, who have long accused Nouri al-Maliki of discriminating against them and monopolizing power.

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Iraqi Islamist militants have invaded the country’s biggest oil refinery, after pounding it with mortars and machine guns from two directions.

An official quoted by Reuters said the militants now controlled 75% of the Baiji refinery, 130 miles north of Baghdad.

Government forces have made new air strikes on militants advancing towards the capital.

Fighting is also reported in the western city of Ramadi.

The government is battling to push back ISIS (Islamic State in Iraq and the Levant) and its Sunni Muslim allies in Diyala and Salahuddin provinces, after the militants overran the second city, Mosul, last week.

PM Nouri al-Maliki appeared on television with Sunni Muslim and Kurdish leaders on Tuesday to issue a call for national unity in the face of the advance – they demanded that non-state forces lay down their arms.

However, such a call is unlikely to have much effect as Nouri al-Maliki has openly sponsored the formation of Shia Muslim militias to fight alongside regular Iraqi troops.

Hundreds of people have been killed since the start of the militant offensive last week, many of them believed to be captured soldiers publicly shot by ISIS-led firing squads.

During fighting in the city of Baquba this week, 44 prisoners were killed inside a police station in unclear circumstances.

The attack on the refinery started at 04:00 a.m. from outside two of the three main entrances to the refinery, according to Reuters.

Smoke rose from a spare parts warehouse and some stores of oil were reportedly destroyed.

“The militants have managed to break into the refinery,” the unnamed official told Reuters from inside the refinery.

“Now they are in control of the production units, administration building and four watch towers. This is 75% of the refinery.”

Foreign personnel were evacuated earlier but local staff reportedly remained in place, with the military defending the facility.

Baiji accounts for a little more than a quarter of the country’s entire refining capacity, all of which goes toward domestic consumption for things like petrol, cooking oil and fuel for power stations, an official told AP news agency.

Militants in the western province of Anbar, where the capital is Ramadi, said they had made advances, with a number of police stations near the town of Hit going over to dissident tribes.

Further north, the Iraqi government said it had recaptured the citadel in the strategic town of Tal Afar, where militants were said to have taken control on Monday.

Using unusually strong language, Nouri al-Maliki accused Saudi Arabia – which is largely Sunni – of backing ISIS. He also fired four army commanders for failing to halt the sweeping advance by the militants. They included the top commander for Nineveh, the first province where ISIS fighters made major gains.