The widely anticipated ousting of ISIS from its stronghold of Mosul in northern Iraq is likely to transform the extremist group into an even more dangerous force, a Canadian general who directs training of Iraqi security forces said Wednesday.

Brig. Gen. Dave Anderson told reporters at the Pentagon he is certain the Iraqis will prevail in Mosul.

"But the fall of Mosul does not mean that Daesh is defeated by any stretch of the imagination," Anderson said, using an alternative acronym for ISIS, militants fighting to establish an Islamist state. "It just means it's defeated in its current format."

Anderson said he is confident that in defeating ISIS in Mosul, the militants will be stripped of their capacity to conduct conventional military operations. Then, however, the group's remaining fighters are likely to melt into the civilian population and morph into an insurgency.

"So it's definitely not over" after Mosul, Anderson said. "If anything, it's gonna be more difficult."

Anderson was speaking from a U.S.-led coalition military facility in Iraq.

The Iraqi government is preparing to launch a major military operation, with air support from the U.S.-led coalition, to retake Mosul this year.

The period between the fall of Mosul and the ultimate defeat of ISIS "is probably when it's most dangerous," he said. He did not say how long he thought it would take to fully defeat ISIS after it loses control of Mosul, but he said intensive planning is underway to help Iraqi forces prepare to fight ISIS in its post-Mosul form.

"Literally what we've been talking about is how do we position police forces and minister of interior forces in order to be able to fight the enemy the day after Mosul and its new metastasized form," Anderson said. "We're working on that pretty hard right now."

Anderson said that once Mosul is declared secure, some Iraqi security forces will be pulled out of the city and retrained and re-equipped to conduct counter-insurgency fights.

He said it is expected to take 30,000 to 45,000 Iraqi security forces to hold Mosul once it has been retaken, "employing local police who will serve as the face of security for Iraq."