Iraqi men line up today to volunteer for military service in Baghdad, Iraq, after authorities urged Iraqis to help battle insurgents. The campaign by the al-Qaida-inspired Islamic State militants has raised the specter of the sectarian warfare that nearly tore the country apart in 2006 and 2007.

BAGHDAD — Sunni militants hung their black banners on watch towers at Iraq’s largest oil refinery, a witness said today, suggesting the vital facility had fallen to the insurgents who have seized vast territories across the country’s north. A top Iraqi security official, however, said the government still held the facility.

The fighting at Beiji, 155 miles north of Baghdad, comes as Iraq has asked the U.S. for airstrikes targeting the militants from the Islamic State of Iraq and the Levant. While U.S. President Barack Obama has not fully ruled out the possibility of launching airstrikes, such action is not imminent in part because intelligence agencies have been unable to identify clear targets on the ground, officials said.

The Iraqi witness, who drove past the sprawling Beiji refinery, said militants also manned checkpoints around it. He said he saw a huge fire in one of its tankers. He spoke on condition of anonymity because he feared reprisals.

The Iraqi security official said the government force protecting the refinery was still inside today and that they were in regular contact with Baghdad. The refinery’s workers had been evacuated to nearby villages, he said.

Helicopter gunships flew over the facility to stop further militant advances, the official said. The insurgents took over a building just outside the refinery and were using it to fire at the government force, he said.

The army officer in charge of protecting the refinery, Col. Ali al-Qureishi, told state-run Iraqiya television by telephone that the facility remained under his control. He said nearly 100 militants had been killed as his force repelled wave after wave of attacks since Tuesday. The country’s chief military spokesman, Lt. Gen. Qassim al-Moussawi, echoed al-Qureishi’s assertion in comments made at a news conference today.

The Beiji refinery accounts for just over a quarter of the country’s entire refining capacity — all of which goes toward domestic consumption for things like gasoline, cooking oil and fuel for power stations. Any lengthy outage at Beiji risks long lines at the gas pump and electricity shortages, adding to the chaos already facing Iraq.