The operation to retake Iraq's second largest city from Islamic State militants will likely begin in April or May, a senior US military official says.
The operation will involve about 12 Iraqi brigades, or between 20,000 and 25,000 troops, the official said on Thursday.
Laying out details of the expected Mosul operation for the first time, the official from US Central Command said five Iraqi Army brigades will soon go through coalition training in Iraq to prepare for the mission.
Those five would make up the core fighting force that would launch the attack, but they would be supplemented by three smaller brigades serving as reserve forces, along with three Peshmerga brigades who would contain the Islamic State fighters from the north and west.
The Peshmerga are Kurdish forces from northern Iraq.
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The official said there also would be a Mosul fighting force, largely made up of former Mosul police and tribal forces, who would have to be ready to go back into the city once the army units clear out the Islamic State fighters.
Included in the force would be a brigade of Iraqi counterterrorism forces who have been trained by US special operations forces. The brigades include roughly 2,000 troops each. The official was not authorised to discuss the operation publicly and spoke on condition of anonymity.
The official said the US will provide military support for the operation, including training, air support, intelligence and surveillance. The official said there has been no decision made yet on whether to send in some US ground troops to help call in airstrikes.
Islamic State militants overtook Mosul last June, as the group marched across large sections of Iraq and Syria, sending Iraqi forces fleeing.
At this point, officials estimate there are between 1,000 to 2,000 Islamic State insurgents in the city of Mosul.
Military leaders have been talking about retaking the city for some time, but they have said they won't launch the operation until the Iraqi troops are ready.
The official said they wanted to retake Mosul in the spring, before the summer heat and the holiday month of Ramadan kick in.
