Story highlights 'Hundreds' of US troops are stationed at a base 40 miles of Mosul

The battle for Iraq's second-largest city could start as soon as October

Washington (CNN) Indications are growing that the attack to retake Mosul, Iraq's second largest city and ISIS' last major stronghold in the country, could begin as soon as next month.

Hundreds of US troops have arrived at an air base 40 miles south of Mosul to support Iraq's efforts to liberate that city, a US defense official told CNN.

The US-led coalition to counter ISIS says it carried out four additional airstrikes near Mosul and Qayyarah Friday, destroying eight ISIS fighting positions and damaging a tunnel entrance. Near Qayyarah, two strikes engaged an ISIS tactical unit and destroyed a vehicle, a weapons cache and 29 watercraft.

Qayyarah air base was recaptured from ISIS by Iraqi soldiers backed by US airstrikes in July and the American forces operating there will mainly provide logistics, supplies and support for the Iraqi offensive on Mosul. The move brings US personnel closer to the battle and ISIS' defensive lines.

"When the (Iraqi Security Force) is ready to move on in their operations to get after Mosul, we'll be prepared to support that and the airfield will be ready," Lt. Gen. Jeffrey Harrigian told reporters Tuesday at the Pentagon. He added that coalition forces are conducting intelligence, reconnaissance and surveillance efforts in the area.