Iraqi forces are nearing what is expected to be the toughest part of the fight for the northern Iraqi city of Mosul. As troops push toward the river dividing the city, they face new tactics from Islamic State fighters adapting to an urban environment and the limitations of U.S. air and artillery support.

The more densely populated west side of the historic city, with its twisting streets and covered market, is still in ISIS hands. But Iraqi counter-terrorism troops this week for the first time reached the east bank of the Tigris River that splits the city.

"We know ISIS very well by now and we knew it would take a long time," Iraqi Gen. Abdul-Wahab al-Saadi told NPR in a recent interview near the front line.

Saadi commands 12 battalions of counter-terrorism forces. He won't say exactly how many troops are under his orders, but several thousand of his fighters have led Iraq's battle against ISIS for the last two years, pushing them back on several fronts, from areas north of Baghdad to Tikrit to Fallujah and now in Mosul.

The last time I saw Gen. Saadi was in Fallujah, as Iraqi forces took back that city in western Iraq last summer.

He says ISIS is putting up a tougher fight in Mosul. The extremist group has held the Iraqi city since it overpowered the Iraqi army and police in June 2014 and declared it the capital of their self-declared Islamic state, or caliphate.

"Their fighters in Mosul are tougher than they were in Fallujah," says Saadi, over tea in an abandoned house in a village near the front line. "In Fallujah, there were just fighters and a few car bombs. Here until now, they have detonated more than 250 car bombs just against my men."

As the fight has moved into more heavily populated neighborhoods and out of the open spaces of the countryside, he and U.S. commanders say ISIS is increasingly using drones.

"They are using them for many purposes," says Saadi. "One of the purposes is to track me."

He says the small drones bought on the market are used to help ISIS snipers and suicide bombers in targeting. ISIS has begun attaching explosives to larger drones.

Mosul was a commercial center, filled with factories and workshops. ISIS has taken machine tool shops and converted them to manufacturing military-grade mortars. It has built car bomb factories across the city and has welded steel plates onto vehicles to turn them into more lethal vehicle-borne bombs.

Saadi shows us one of the locally made mortars. It's almost indistinguishable from Iraqi army mortars apart from the black and white ISIS logo painted on it.

The Iraqi counter-terrorism forces, trained by the U.S., are considered by the U.S. military to be the most professional in the Iraqi forces. Iraqi counter-terrorism troops are backed directly by U.S.-led air support. The Iraqi army and federal police are supported by Iraqi air strikes.

But as the fight moves further into the city, air strikes and artillery have limited use.

"Hospitals, mosques, schools, churches – those are all the places the US is not supposed to hit and all the places that ISIS is hiding in," says Saadi.

Although Iraq and international organizations have prepared camps for a flood of displaced Iraqis, relatively few have been able to leave. More than 700,000 civilians are still living in west Mosul.

In last year's battle for Fallujah, military leaders believe ISIS fighters escaped with thousands of civilians, all of them fleeing when the Iraqi army opened up a corridor.

In Mosul, Iraqi forces have surrounded the city and the battle plan has allowed no escape for anyone. Civilians now running out of food and water will be able to leave only as their neighborhoods are liberated.

"The civilians are a problem," says Saadi. "Their position near the front line makes us a little worried. It means we go slowly."

Near the airport in the Kurdish capital Erbil, the deputy commander of U.S. forces in Iraq tries to explain the complexity of the fight for Mosul.

"Picture any large metropolitan city on the U.S. East Coast — dense, older cities with smaller streets. And then picture having to eradicate all crime and any enemy force in there," says Brig. Gen. Scott Efflandt.

"It requires street by street, house by house, room by room operation," he said. "There's no quick way to do it. You have to walk, you have to climb stairs, you have to open doors and then repeat the process again and again and again and then when you're doing that you have to leave someone behind to guard the area you just went through. "

That's without U.S. troops on the frontlines.

After the U.S. and its allies invaded Iraq in 2003, American forces spent eight years in Iraq. Mosul is the biggest urban battle involving the U.S. in years. However, unlike previous battles, most of the 5,000 U.S. troops in Iraq aren't anywhere near the line of fire.

"When we think of what we do in Iraq, it's very difficult for people to understand we are not side by side with an Iraqi soldier shooting a rifle. It's not what we are doing here," says Col. Brett Sylvia, commander of the 2nd brigade, 101st Airborne Division – a force whose soldiers actually walked the streets of Mosul early in the war.

Sylvia commands about 2,500 troops advising and assisting the Iraqi military, mostly at tactical operations centers.

At Camp Swift, the small U.S. base outpost near Makhmour in northern Iraq where Col. Sylvia is based, a steel door connects the base to the Ninevah Operations Command. Iraqi generals sit next to U.S. officers looking at surveillance feeds from coalition aircraft .

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