In an unusual move,
A senior official with U.S. Central Command, who briefed reporters on condition of anonymity, said the operation would involve as many as 25,000 Iraqi government troops and Kurdish militia fighters backed by coalition airstrikes and surveillance systems, and could begin in April or May.
It wasn't immediately clear why the Pentagon would publicly disclose a major military operation so far in advance unless it was intended, at least in part, to rattle the estimated 2,000 Sunni fighters believed to hold the northern Iraqi city.
Islamic State calls Mosul, a city of 1.4 million people, a capital of its self-declared caliphate, or Islamic empire. Retaking it from the militants and their supporters could involve bitter house-to-house fighting in a crowded urban environment.
"Mosul will not be easy," the U.S. official said. "It's going to be a difficult fight."
The official said Central Command hopes to launch the attack before the heat of summer and before the June 17 start of the Islamic holiday month of Ramadan.
"By the same token, if they're [Iraqi forces] not ready, if the conditions are not set, if all the equipment they need is not physically there and they [are not] trained to a degree in which they will be successful, we have not closed the door on" delaying the mission, the official said.
Iraqi army units in northern and western Iraq largely collapsed when Islamic State fighters stormed across the country last spring and summer. Forces loyal to the government in Baghdad have yet to retake any major cities or towns, though sectarian Shiite militias have recaptured some areas.
But the U.S. official who briefed reporters said the Pentagon had identified a dozen or so Iraqi army brigades that can be deployed to push the militants out of Mosul. About five brigades would lead the initial attack, he said.
Iraqi special forces and former Mosul police officers would also participate in the operation.
The official said planners have not decided whether U.S. military advisors should accompany the Iraqi ground troops assaulting Mosul to call in airstrikes by the U.S.-led coalition.
"The full range of support that we have given them to date is the baseline," the official said. "It's a matter if we take it one level higher" with spotters to direct airstrikes and special operations teams for more precise bombing runs.
Several thousand U.S. troops are coordinating airstrikes and training troops in Iraq, but the Obama administration has not allowed any to accompany Iraqi or Kurdish ground troops on combat missions so far.
Kurdish fighters backed by coalition airstrikes have cut some Islamic State supply lines near Mosul in recent weeks. The Kurdish fighters, known as peshmerga, hold territory north and west of Mosul.
Warplanes from the U.S.-led coalition have carried out more than 150 airstrikes in and around Mosul since August.