Mosul Is Just the Beginning, and It Will Only Get Tougher: U.S. Commanders

A senior U.S. military official warns that ISIS fighters in Mosul are likely to resort to even more extreme and harsh actions as they face more and more military pressure on the city.

"Unfortunately, ISIS will likely grow more oppressive to the population in areas they control," the official said, adding, "they will either leave or they will be more oppressive and force folks to fight that shouldn't fight."

"I imagine we will see some of that as we get closer to Mosul," the official said, "ISIS comes down and comes down hard."

In an exclusive interview with NBC News, the head of U.S. Central Command, General Joseph Votel, warned that ending an ISIS reign over the city of Mosul will not end their presence in the country

"Mosul, while it's absolutely essential to complete that operation successfully, it will not mean the end of the Islamic State in Iraq," Votel said.

The general said that re-claiming Mosul from ISIS would "take away a significant area for them," but he added that, "we should expect that they will adapt as they always have. "

Votel acknowledged that an ISIS defeat in Mosul will likely morph the terrorist group into an insurgency.

"Maybe they will move to other areas and begin to take on some of the characteristics of the more traditional terrorist organizations where they aren't beholden to holding terrain and they just focus on doing attacks or other things that are more typical to them."

At the same time, pushing ISIS out of Raqqa will not put an end to the group in Syria either.

"There is another phase that comes after this," Votel said. "The phase of the operation that we're in right now I describe as dismantling the caliphate. We're taking apart the physical caliphate."

"And then we will move into the defeat phase here, which will be a much more deliberate effort to working with our partners, Iraqis and our partners on the ground in Syria, to eliminate what remains."

As for when the operation will begin to re-take Raqqa, a senior U.S. military official said that the forces are trained and ready to begin to isolate the city, but they may not be ready to begin the actual assault.

"I know we've got the forces to isolate it," the official said, but quickly added that there are too many unknown factors to be sure that there are enough forces to actually assault the city.