Hundreds of additional U.S. troops have flowed into Iraq during the past week as American and Iraqi forces there begin final preparations to launch an invasion of Mosul this fall.



The size of the U.S. force in Iraq and Syria now tops 4,400, up from about 3,900 last week, defense officials said.



President Obama authorized several troop increases for Iraq earlier this year but those troops did not deploy immediately. The latest uptick brings the current footprint closer to the legal cap of 4,647.



Air Force Col. John Dorrian, a top spokesman for U.S. forces in Iraq, declined to say what those troops are doing but said preparations are underway to potentially launch the Mosul invasion this fall.



"There's a tremendous amount of work going on to set conditions, including the logistics detail that would be required in order to go after Mosul. And then we continue to hammer the enemy with strikes, including both artillery and airstrikes," Dorrian told reporters Thursday at a press briefing.





Intelligence estimates suggest between 3,000 and 4,500 Islamic State militants are prepared to defend Mosul. The Iraqis are planning to gather between eight and 12 brigades for the invasion.

The U.S. forces in Iraq are authorized to provide combat advice to brigade- and battalion-level Iraqi forces and possibly fly U.S. attack helicopters to provide close-air support for Iraqi troops.

Army Lt. Gen. Stephen Townsend, the commanding general in Iraq, told the Wall Street Journal this week that the invasion of Mosul will begin in October.