Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of "United States of Jihad: Investigating America's Homegrown Terrorists."

Qayyarah Airfield, Iraq (CNN) US military officials expect that senior leaders of ISIS may try to flee Mosul, the capital of ISIS' self-styled caliphate in Iraq, as their control of the city comes under attack from Iraqi forces, backed by the US.

ISIS leaders will likely take women and children as human shields, or will pose as refugees leaving the battle for the city, according to those officials.

Fleeing ISIS leaders will probably either slip across the border into neighboring Syria or to the area around Al Qaim, a remote desert town on the Iraq-Syria border.

Leading up to the Mosul assault, over the past several months the US military has killed 36 leaders of ISIS, according to a US military official.

Another American officer says that the "the life expectancy of a military leader in Mosul is relatively short." Successful attacks on leaders sow chaos among the ISIS rank and file who tend to coalesce around their charismatic leaders.

ISIS under attack

ISIS' whole enterprise also is under attack in Mosul from US drones, manned aircraft and Apache helicopters. Their targets include IED construction facilities; the sources of ISIS financing, such as oil-related facilities, and also the group's media production entities, such as ISIS' news agency, Amaq, according to US military officials.

It is not clear whether ISIS' overall leader, the self appointed caliph, Abu Bakr al-Baghdadi, is in Mosul or elsewhere, according to a US military official.

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ISIS has had two years to prepare the defenses of Mosul, which include berms, trenches and "quite extensive" underground tunnels, some as long as 2 kilometers, according to a US military official. Giant concrete T walls have also been laid down on their sides and intermingled with booby traps and bombs to impede vehicles moving into the city.

Some houses in the Mosul defensive zone have been elaborately booby-trapped; for instance, light switches inside some houses have been wired so they detonate.

US Army Lt. Gen. Stephen Townsend, who commands the anti-ISIS coalition, says, "We see the defense around the city, as a hard center, a softer middle and a very hard crust. And the Iraqis right now are working hard on punching through that external crust."

In the past week or so of the Mosul operation, Iraqi forces and Kurdish militias supported by American Special Forces advisers have advanced to within 15 to 20 kilometers from the city, which is being attacked from multiple directions. These forces have captured 31 ISIS-held towns and villages surrounding Mosul, according to American military officials.

There are a total of 4,500 to 7,500 ISIS fighters defending the city, of which some 1,000 are believed to be foreign fighters, who are often the most ideologically committed in the organization.

At some point US officials expect that ISIS fighters will fall back to Mosul city itself, where the terrorist army has "canalized" streets so that Iraqi forces are drawn into their fields of fire.

Leaving by the back door?

US military officials believe that around this time some ISIS leaders will "go out the back door," shaving off their beards and blending into the local population.

Some hardcore Iraqi ISIS fighters, as well as the estimated 1,000 "foreign fighters" from around the Muslim world, are expected to fight to the death in the city. Gen. Abraheen Tahseen al Khafaji, a spokesman for the Iraqi Ministry of Defense, says that the foreign fighters have "no ability to return to their home countries" elsewhere in the Arab world or Europe and therefore are more likely to fight to death.

The forces attacking ISIS in Mosul have largely encircled the city from the north, south and east.

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There is a gap in the encirclement to the west of the city, which seems to have a purpose: to draw out any ISIS fighters who do not want to fight to death in the heavily populated city. If ISIS fighters retreat from the city heading west across the desert to Syria, American surveillance drones can readily detect them.

So far, US military officials have seen no large group of ISIS fighters try to leave Mosul, nor have they seen a large influx of ISIS fighters.

As the battle for Mosul continues, US military officials expect to seize a large intelligence haul from the operations that will help with planning the next phase of the operation, which is to move on to the remaining ISIS strongholds in Iraq such as Tal Afar and Al Qaim. They say they are also "deep in the planning" of the operation to take Raqqa, ISIS' de facto capital in Syria.

Tightening the noose

ISIS just got some more bad news: Qayyarah Airfield, not far from Mosul, is now able to land large American and Iraqi fixed-wing aircraft. Qayyarah Airfield is a former Iraqi air force base dating from the 1970s that is only 60 kilometers from Mosul.

Recognizing its strategic importance, after ISIS militants seized Qayyarah airfield two years ago, they set about destroying it so that it couldn't be useful for anyone seeking to overthrow them, according to US military officials. ISIS fighters used heavy construction equipment to dig deep trenches into the runway, rendering the airfield unusable. They also destroyed the airport tower.

However, in July ISIS lost the airport to Iraqi forces and a month later American engineers arrived to repair the 5,000-foot runway.

The engineers sealed up the trenches that pocked the airfield, filling them with gravel and then resurfaced the runway.

The first American fixed wing aircraft to test the field flew in to Qayyarah on Sunday.

Underscoring the airfield's strategic importance, the third plane to arrive at Qayyarah, on which I traveled, carried Gen. Joseph Votel, the commander of U.S. Central Command who oversees the wars in Iraq and Syria.

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Votel flew in to Qayyarah on Tuesday in a C-130. On the airport tarmac, the commander said that the restoration of the airfield was "extraordinarily important" as it would dramatically increase the ability to resupply the Iraqi forces.

The US military now terms Qayyarah airfield, Q-West.

Accompanying Votel was Townsend, who said that Q-West plays an important role for the battle for Mosul because Iraqi commanders and their US advisers are now "commanding and controlling the fight from here."

As the campaign against ISIS reaches a climax with the massive operation to take Mosul, Q-West will prove invaluable as the U.S. and Iraqi militaries now have the capacity to fly in massive amounts of food, medicine and ammunition to resupply their advancing forces.

The prognosis for the ISIS fighters who remain in Mosul is not good, although both US and Iraqi military officials believe that the final phase of the fight for the city will be brutal.