A senior U.S. military official says the operation to retake Iraq's second largest city from Islamic State militants will likely begin in April or May and will involve about 12 Iraqi brigades, or between 20,000 and 25,000 troops.
The unnamed official from U.S. Central Command said five Iraqi Army brigades will soon go through coalition training in Iraq to prepare for the mission.
Those five brigades -- the core fighting force that would launch the attack -- would be supplemented by three smaller brigades serving as reserve forces, along with three Kurdish Peshmerga brigades.
Included in the force would be a brigade of Iraqi counterterrorist troops who have been trained by U.S. special forces. The brigades include roughly 2,000 troops each.
IS militants overtook Mosul in June, as the group marched across large swathes of Iraq and Syria.
The official was not authorized to discuss the operation publicly and spoke on condition of anonymity.
The official said the U.S. will provide military support for the operation, including training, air support, intelligence and surveillance.
Officials estimate there are between 1,000 to 2,000 Islamic State insurgents in the city of Mosul.