A military offensive to retake Iraq's second largest city from the Islamic State of Iraq and the Levant (ISIL) will likely begin in April or May, a senior US military official has said.

Laying out details of the expected operation in Mosul for the first time, the official from US Central Command on Thursday said five Iraqi Army brigades would 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 battle ISIL from the north and west.

The Peshmerga are Kurdish forces from northern Iraq. 

The Pentagon official said the battle would involve at total of about 12 Iraqi brigades, or between 20,000 and 25,000 troops.

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 ISIL 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.

US military support

The official said the US would 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. 

ISIL 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 ISIL fighters in the city of Mosul. Military leaders have been talking about retaking the city for some time, but they have said they will not 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.

"But by the same token, if they're not ready, if the conditions are not set, if all the equipment they need is not physically there and they [aren't] trained to a degree in which they will be successful, we have not closed the door on continuing to slide that to the right,'' he said.

The official also revealed for the first time that Qatar has agreed to host a training site for coalition forces to train moderate Syrian rebels who would return to Syria to fight ISIL.

Other sites are in Turkey, Jordan and Saudi Arabia.

Source: AP