Fierce fighting has erupted at Iraq's largest oil refinery after militant fighters with the so-called Islamic State (IS) used suicide bombers, car bombs, rockets, and mortars to attack the small contingent of Iraqi government forces charged with defending the sprawling facility.

"The refinery is highly contested, and there's a committed effort by [IS] to take it over," a senior US military official told VICE News about the fight at the Baiji refinery, about 150 miles north of Baghdad. "The Iraqis do not have the upper hand in this, it's stalemated."

The fighting at the Baiji refinery has continued, on and off, for more than a year. Islamic State originally captured the 14-square-mile refinery compound when the group first surged into northern Iraq in June 2014, but Iraqi security forces later recaptured much of it in November.

The US official, who spoke to VICE News on condition of anonymity, expressed frustration with the Iraqis' inability to go on the offensive and push IS out of the 14 square mile compound.

"They do not have the ability to maneuver, they do not have the ability currently to regain lost terrain north of Baiji, and to re-establish the [refinery] perimeter and hold it so that additional[IS] fighters do not continue to breach it every time you turn around," he said. "Yes, it's tough and hard, that's their mission. They've gotta have the balls, you can't hide behind a goddamn rock."

"We'll roll in with air strikes, hellfire missiles, and kill them," he said. "But air power alone is not going to win this."

Iraqi Security Forces are spread thin across the country due to recent offensives against IS in Tikrit and Ramadi. The Iraqis have met with some success, and, with the help of Iranian-backed Shiite militias, successfully recaptured Tikrit from IS last month.

An IS victory at the Baiji refinery would constitute a serious military and symbolic blow to the Iraqi government and security forces. It would also give IS a new revenue source if the group was able to refine and sell oil from the facility.

US-led coalition airstrikes at Baiji had been "clearly decisive" in stopping IS's advances, the US military official said, adding that there were strikes on Friday and Saturday at the facility.

Several reports have indicated that IS has surrounded and trapped the Iraqi defenders inside the oil refinery after hundreds of militants overran government forces' positions late last week. One Iraqi officer told McClatchy news service on Saturday that his forces were under siege, and were low on ammunition, food and water.

"All of us now are thinking of committing suicide," an Iraqi federal police officer in the facility told the independent news site Iraq Oil Report on Thursday night. "We have very little food and ammunition,and we can't withstand the suicide bombers, snipers and rockets."

"We'll roll in with air strikes, hellfire missiles, and kill them. But air power alone is not going to win this."

IS has made a show of beheading and executing Iraqi security forces in the past, including a video published last week by IS that claimed to show the group's fighters overrunning parts of the Baiji refinery and Iraqi security forces' positions.

In the video, black smoke billows from burning oil storage tanks as IS fighters execute Iraqi government troops, pose in front of destroyed Humvees and armored vehicles, and load American-made weaponry and ammunition into the beds of pickup trucks.

Islamic State video via Long War Journal

The insurgency news and analysis site Long War Journal posted the IS video, pointing out that it couldn't confirm when the video was shot or produced, since it contained some new scenes interspersed with older footage that had already appeared in IS videos about the battle at the refinery.

While not discounting the severity and seriousness of the Baiji fighting, the senior US military official gave a less grave assessment of the Iraqis' situation. He said the facility is not in danger of falling to IS, for now, and Iraqi forces were "putting the appropriate effort towards it."

The US official could not specify exactly how much of the facility the Iraqis currently control. IS propaganda claims the group controls 90 percent of the facility, but intelligence analysts who spoke with VICE News believe that number is inflated.

The US military official also said that while the refinery compound, which, at 14 square miles is 10 times the size of Manhattan's Central Park, is not technically surrounded by IS, the roads into the facility are lined with booby traps and explosives.

He said the Iraqis had flown two resupply missions, including food, water, medicine, and ammunition, to their forces at Baiji during the past week, using helicopters and C-130 aircraft.

"If they get overrun, they are all likely going to die, hence talk of committing suicide, before they get captured by ISIS and beheaded or burned in a propaganda video."

Ben Lando, founder and Iraq bureau chief of Iraq Oil Report, said Iraqi government security forces told him that they had been "hampered by inconsistent supply lines and incoherent strategic planning," and that there was no clear plan to back up or rescue the government's forces at the site.

Moreover, in the event IS were to overrun the refinery, it's not clear if the Iraqi security forces defending the facility could be evacuated.

"There are egress routes south out of the refinery, especially if the [Iraqi Security Forces are] holding the [southeast] gate," said Landon Shroder, a civilian intelligence analyst who focuses on Iraq.

"These units should be able to withdrawal south along Highway 1, in the direction of Tikrit, unless they are 100 percent boxed in."

"If they get overrun, they are all likely going to die, hence talk of committing suicide, before they get captured by ISIS and beheaded orburned in a propaganda video," he added.

Shroder said evacuation by helicopter would also be "extremely difficult" given that IS controls significant portions of the refinery compound and the surrounding area.

Related: Related: Video Shows Islamic State Fighters 'Buying and Selling Yazidi Slaves'

He said the government's possession of the refinery is "critical for national morale," and he expected the Iraqis and US-led coalition to increase the number of air strikes if the Iraqi defenders were in danger of being overrun.

In addition to the possible human cost of an IS victory at Baiji, Lando worried about the effects on Iraq's oil industry in the event IS captured the facility.

"It risks ultimately Iraq's largest refinery being damaged by an attack, or IS scorched-earth types of actions, and potentially ruining the refinery for good," he said.

In mid-April, IS mounted an offensive that saw the group's fighters blast through a perimeter wall and capture several storage tanks and other buildings in the complex, Reuters reported.

There are currently 3,100 US military personnel in Iraq helping to train the Iraqi security forces. None are at the Baiji refinery, the US military official said.