WASHINGTON — 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, a senior U.S. military official said Thursday.

Laying out details of the expected Mosul operation for the first time, the 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 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.

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 U.S. special operations forces. The brigades include roughly 2,000 troops each. The official was not authorized to discuss the operation publicly and spoke on condition of anonymity.

Military leaders don’t often disclose as many details of an operation before it takes place, but in some cases it can have an impact on the enemy, trigger a reaction or even prompt some militants to flee before the assault begins.

The operation itself comes as no surprise to the Islamic State group. Iraqi leaders have for months made it publicly clear that they were planning an operation to retake Mosul and that they were eager to get started. In addition, U.S. officials had already acknowledged that they were beginning preparations for the Mosul mission, including using airstrikes to shut down supply lines that the insurgents were using to get equipment or people in and out of the city.

Asked why U.S. Central Command was telegraphing the timeframe and details of the operation to the enemy, the official said it was important to highlight the effort the Iraqi security forces are putting into the mission and how committed they are to it.

The official said the U.S. 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 U.S. ground troops to help call in airstrikes. That decision would be made by senior defense and military leaders and President Barack Obama.

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.

“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.

Under the plan, the approximately 3,200 Iraqi forces that have completed the training already or are going through it now would replace the five main brigades wherever thay are now, and those five units would then go through several weeks of final training before the Mosul operation begins.

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 the Islamic State forces there. Other sites are in Turkey, Jordan and Saudi Arabia.

The training facility in Jordan is ready to go. The technical agreement on the Turkey training site was signed Thursday, and it is nearly ready. The facility in Saudi Arabia will be ready to open in one to three months, and the site in Qatar will be finished in six to nine months, the official said.

The U.S. and other coalition nations will train the Syrian fighters so they can return to their own country and battle the Islamic State group.