The Pentagon says the Islamic State is on the verge of defeat in Iraq, and that after Mosul is liberated, ISIS will control only small towns and large swaths of empty desert.

Pentagon spokesman Capt. Jeff Davis said the announced deployment of 615 U.S. troops to two bases in Iraq is the final force adjustment anticipated before the Mosul offensive, which could begin as soon as early October.

"This is the number of forces we think we need to enable Iraq to do its next major objective which is Mosul," Davis said. "To be frank, there are no major objectives after that."

Asked if the fall of Mosul is tantamount to the defeat of ISIS in Iraq, Davis said "Yeah, certainly. This is it. Mosul is the last major bastion that they hold, and once it is clear, and it will be clear, what's left is going to be significantly less difficult."

He said an ISIS ejection from Mosul leaves only small towns and areas of the desert to be secured, which would not require additional U.S. forces.

Discussions about sending the additional forces have been in the works for weeks, but officials say Iraq has signed off on the reinforcements.

Defense Secretary Ash Carter decribed the pending deployment week as an opportunity "further to accelerate the defeat of ISIL in Iraq."

"Obviously the taking of any of those opportunities would need to be, as we do everything in Iraq, with the permission of the government of Iraq, and in particular with Prime Minister Abadi," he said.

If all 615 troops are deployed, it would put the total U.S. forces in Iraq at 5,262. The troops specialize in logistics, training and intelligence.

Davis said some of the additional troops will go to Qarrayah West, and the remaining will be deployed to Al Asad Air Base. It will be the final planned addition of U.S. troops ahead of the Mosul offensive, although Davis would not rule out further increases.