The tough fight ahead to retake Western Mosul

— -- This past weekend, Iraqi military forces began the assault to retake the western half of Mosul from ISIS in what is expected to be a tough fight.

It took Iraqi military forces 100 days of street-to-street fighting to finally retake the eastern half of Mosul, Iraq's second largest city, but U.S. military officials anticipate that the fight to retake the western side of the city could be even more difficult.

The western side of Mosul, on the left bank of the Tigris River, is more densely populated than the eastern side and it is believed that ISIS fighters will take advantage of the narrow streets to slow down the Iraqi military offensive.

Here's a look at how the second phase of the battle for Mosul could shape up.

A Tough Fight in Western Mosul

"We do expect it to be an extraordinarily difficult fight" Col. John Dorrian, the spokesman for Operation Inherent Resolve, told Pentagon reporters Wednesday. "The enemy has not given up."

According to Dorrian, the U.S. military believes that between 1,000 and 3,000 ISIS fighters are currently in western Mosul hiding among an estimated 750,000 civilians remaining in the city.

"We do expect it to be a very tough fight because the very narrow areas, the very narrow streets in some parts of the city, the ancient parts of the city, are going to make for a very tough going," said Dorrian.

The narrow streets will limit the Iraqi military's ability to use vehicles in their assault on the city.

But they will also likely prevent ISIS from launching the deadly suicide car bomb attacks it used to slow down the Iraqi military in eastern Mosul. The car bomb attacks resulted in significant casualties among the Iraqi military's elite Counter Terrorism Service that was doing most of the intense fighting in eastern Mosul.

Iraqi military forces are expected to face even tougher ISIS resistance in western Mosul. Dorrian noted that there were roughly 100,000 buildings in eastern Mosul that had to be cleared by the Iraqi military and that there are a similar number of buildings on the western side of the city in an even more compressed area.

Dorrian said Iraqi forces will face a tough fight because each of "these buildings have to be cleared from rooftop level through every room, every closet, all the way down to ground level, including the tunnels that get dug between buildings."

"It's very, very dangerous and tedious, and the Iraqi security forces have done a really good job of protecting civilians as they've conducted those clearing operations and that's something we expect them to continue." said Dorrian.

What Will the Offensive Look Like?

The offensive for western Mosul has begun with Iraqi forces pressing northward to the southern stretches of the city. In the three days since the start of the offensive, they have already taken back 48 square miles and are now overlooking the city's airport.

It is expected that the Iraqi military will face tougher ISIS resistance in the fight for the airport.

The offensive is being led by the Iraqi Army's Ninth Division and the Iraqi Federal Police who are leading the offensive into western Mosul. It was the emergence of the Iraqi Federal Police in late December that helped turn the tide in eastern Mosul. It is expected that forces from the Counter Terrorism Service will once again play a key role in the push into western Mosul.

For months, Shiite militias have pushed northwest of the city to cut off the main road from Mosul to Tal Afar, another ISIS-controlled city. They are there to block the escape of ISIS fighters to that city.

With the Tigris River to the east blocking possible escape routes as well, ISIS fighters will be effectively encircled in the city's western half.

The battle for Mosul has also led American troops to come closer to combat situations even though they are still required to be at Iraqi unit headquarters beyond enemy lines.

Those restrictions have been less applicable to American special operations forces accompanying their Iraqi counterparts, since those Iraqi commanders are always close to the front lines.

But Dorrian explained Wednesday that other American advisers working with commanders of regular Iraqi Army units are "close enough to direct the battle,” he said, adding: " I don't want to give you the impression they're far removed from the front.”

Americans were close enough at times, Dorrian said, that they took enemy fire and found themselves in a combat situation where they had to fight back. He would not disclose whether any American forces had been wounded by enemy fire in such situations.

American advisers assisting in calling in airstrikes targeting ISIS are also closer to the battlefield. "They're not removed from the front, they're very close to the front, close enough to observe what's going on and provide good advice and assistance,” said Dorrian.

It remains unclear if the fight to retake western Mosul will be helped by additional U.S. support that the Trump administration will soon begin to consider.

On Jan. 28, President Trump tasked the Pentagon to lead a review of the strategy against ISIS and to look for new ways to speed up the fight against the terror group. A Pentagon spokesman said Tuesday that the options could be presented to the White House early next week.