Qayyarah Airbase, South of Mosul, Iraq — The American ground force commander in Iraq, Maj. Gen. Gary Volesky, said U.S.-backed Iraqi forces have momentum on their side in the battle to drive ISIS out of its Iraq stronghold, the northern city of Mosul.

Volesky, speaking to NBC News in his first interview since the Mosul offensive began earlier this week, expressed confidence that the terror group will lose Mosul and with it, its caliphate, or Islamic state, in Iraq.

"ISIS has said, this is the crown jewel of Iraq and its idea of a caliphate," Volesky said. "There's not going to be a caliphate, if there ever really was one. So for ISIL, this is going to be a key loss for them and it will be a loss"

Volesky praised Iraqi troops for advancing quickly toward Mosul and said so far the resistance they have meet has been moderate.

"Iraqis have the momentum," he said. "They know it and they want to get there as quickly as they can."

The Iraqi army has a lot riding on the success of the Mosul operation. ISIS was able to swiftly take control of Mosul two and a half years ago when the Iraqi army collapsed in the city, running from the fight and abandoning their weapons.

Volesky doesn't expect that to happen this time.

"They're much more confident and they've learned from these last two years," he said. "Make no mistake, they've learned."

Volesky estimates there are between 3,000 to 5,000 ISIS fighters in Mosul, but he said some have been leaving.

"I'll tell you there are a lot fewer Daesh today than there were yesterday. There will be fewer tomorrow than today," he said, referring to another name by which ISIS is sometimes called.

Despite the successes thus far, Volesky said it's just too early to know how long the Mosul operation will take. Most estimates have ranged from a few weeks to several months.