Kurdish peshmerga fighters take position against the Islamic State on the outskirts of Mosul last month. (REUTERS/Azad Lashkari)

A major military assault to take back the sprawling Iraqi city of Mosul from Islamic State militants could begin in April or May, after more Iraqi troops get training from U.S. advisers, a senior U.S. military official said Thursday.

The official, with U.S. Central Command, said in a conference call with reporters that smaller operations to lay the groundwork for the mission already have begun. The assault itself would amount to the largest ground offensive against the Islamic State since the group swept across much of Iraq last year.

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“The mark on the wall that we are still shooting for is still the April, May time frame,” the U.S. official said, speaking on condition of anonymity. “There are still a lot of things that need to come together.”

The official said that if Iraqi troops are not ready, the operation still could be delayed. But the plan is to launch the offensive no later than May, before the heat in Iraq becomes stifling. Disclosing the plan now, the official said, is part of an effort by the U.S. military to show that Iraqi troops are “absolutely committed” to assaulting Mosul, which was taken by militants in June.

There are believed to be about 2,000 Islamic State fighters in Mosul, home to more than a million people before the militants seized territory across the country  last year. The city, Iraq’s second largest, has been the site of numerous atrocities since, and serves as the group’s de facto capital in Iraq.

A Kurdish peshmerga fighter on the outskirts of Mosul on Jan. 26. (REUTERS/Azad Lashkari)

The senior military official said he expects the force for the Mosul operation to include 20,000 to 25,000 Iraqi troops. The main attack component would include about five brigades of about 2,000 soldiers each, with three additional reserve units, three Kurdish peshmerga units, a brigade of Iraqi counterterrorism troops, and a “Mosul fighting force” comprising mostly former police in the city.

“The five Iraqi army brigades will all go through our training sites before we commence the operation on Mosul,” the official said of the main attack force.

On the Syrian side of the border, U.S. officials continue to hammer out agreements that would allow for the training of moderate rebels. An agreement with Turkey was reached Thursday, and another with Jordan will be finalized soon, the senior military official said. Training also is planned for Qatar and Saudi Arabia eventually.

The United States continues to lead airstrikes against the militants in both Iraq and Syria. As of late Feb. 17, the coalition had carried out a total of 1,383 in Iraq and 1,094 in Syria. Of those, coalition countries had carried out 392 in Iraq and 88 in Syria, U.S. military officials said.

There are presently about 2,635 U.S. troops in Iraq. About 800 are protecting American people and facilities, and an additional 1,835 are involved in the effort to support Iraqi forces.