What U.S. allies are up against in the methodical battle for Mosul

NEAR MOSUL, Iraq -- The U.S.-backed battle for Mosul began to the east of the city, where Iraqi Kurdish fighters went house to house on Monday hunting down the handful of Islamic State of Iraq and Syria (ISIS) fighters still trying to hold out.

In the recaptured town of Tarjala, they showed CBS News correspondent Holly Williams the tunnels ISIS militants had built to defend themselves -- and the aftermath of the four U.S. airstrikes that destroyed them.

Colonel Delshad Mawlod told CBS News the extremists fought to the death, and he had expected to find a tunnel network.

“It’s an ISIS tactic,” he told Williams, “suicide car bombs and tunnels is how they fight.”

These towns and villages to the east of Mosul used to be farming communities, but most of the residents fled as ISIS moved in two years ago. Now it’s an apocalyptic landscape, dotted with oil fires set by the militants.

In the days before the offensive began, the U.S.-led coalition softened the ground with airstrikes inside the city of Mosul.
 
The coalition has released videos showing strikes on what were allegedly ISIS weapons facilities.

Inside Mosul there are now thought to be fewer than 5,000 ISIS fighters -- but also about 1 million civilians.

ISIS is preventing them from leaving, using them as human shields. But some are managing to escape, and they flee to increasingly overcrowded camps around the city.

Hussein Moussa Abdullah is an English teacher who fled the town of Qayyarah, just south of Mosul.

“You cannot sleep well without the fear… the fear is you cannot guess when they will take you from your bed and kill you,” he told CBS News of life under ISIS.

The Iraqi military and its U.S. backers have said the fight to retake Mosul could take weeks, or even months.