The top U.S. commander in Baghdad is confident that Iraqi forces can oust Islamic State militants from their stronghold in Mosul, but he warned that the extremist group may soon re-emerge as an insurgency that continues to mount attacks and destabilize the region.

"I'd like to register a note of caution," Army Lt. Gen. Sean MacFarland, the commander of Joint Task Force Operation Inherent Resolve, said Wednesday.

"Military success in Iraq and Syria will not necessarily mean the end of Daesh. We can expect the enemy to adapt, to morph into a true insurgent force and terrorist organization capable of horrific attacks," MacFarland said, using an Arabic term for the Islamic State group, which is also known as ISIS or ISIL.

Speaking to reporters at the Pentagon, MacFarland declined to speculate about when the American-backed Iraqi forces will mount a full-scale invasion of the city, saying only that "we're going to try to get Mosul back as fast as we can." Some experts believe the operation may begin before the end of this year.

Yet MacFarland is thinking about what happens after the invasion and urging the Iraqis to shift their training accordingly.

For the past year, the U.S. train-advise-and-assist mission has focused on conventional warfare skills, like "combined arms maneuver training, teaching the Iraqis how to integrate infantry, armor, artillery, engineers, aviation and other combat multipliers to achieve an overwhelming advantage at the right place and time on the battlefield," MacFarland said.

But now, MacFarland said, the American advisers have "stepped up our emphasis on police training and recruiting. … The police forces in particular are getting the type of training they need to be a hold force."

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"These men will be key to holding the gains. … We have plans to train quite a number of the additional forces for that mission," MacFarland said.

Holding the city will require thousands of Iraqi police, the general said.

With American air power, artillery and intelligence support, the conventional defeat of ISIS is not in question. But there's growing concern about the battle's immediate aftermath and the transition from the "clearing forces" that invade the city and the "hold forces" that will provide long-term security.

"We'll make sure that we don't pull those clearing forces out too soon, and we'll try to get as many hold forces in there as quickly as we can. But we want to make sure that they have the training and the equipment that they require to succeed," MacFarland said.

Attacking ISIS in Mosul has been the focus of the American military strategy in Iraq for nearly two years. But many experts warn that planting an Iraqi flag in the city will be a short-lived victory.

"The honeymoon in Mosul is going to be extremely brief," said Michael Rubin, a former Pentagon official who is a scholar at the American Enterprise Institute.

"The big question is how are you going to hold it?"