A senior U.S. military official disclosed on Thursday a working timeline for when Iraqi troops may assault Mosul, a city of more than 1 million people that has been under the control of the Islamic State since June. It is expected to be a signature battle against the militant group: Mosul is Iraq’s second-largest city, and there are believed to be at least 2,000 enemy fighters there.

The senior official, speaking to a room full of reporters at the Pentagon, said the assault could begin in April or May and will require between 20,000 and 25,000 Iraqi troops, with the United States in a still-unspecified support role. That has prompted backlash from some observers, including Sen. John McCain (R.-Ariz.), the chairman of the Senate Armed Services Committee.

“It was deeply disturbing to read today than an official from U.S. Central Command, in an official briefing to the media, provided detailed operational information regarding coalition plans to retake Mosul from ISIL,” McCain and Sen. Lindsay Graham (R.-S.C.) said in a letter to President Obama on Friday. “Never in our memory can we recall an instance in which our military has knowingly our own war plans to our enemies.”

McCain and Graham added that the disclosure could put the lives of U.S. and Iraqi troops in danger, and that they want to know who provided the media briefing and whether they had White House permission to release the information.

It’s a stretch to blame Obama when a senior U.S. military official releases details about a plan formed in conjunction with Iraqi officials. But the tweets above provide a pretty good sampling of the knee-jerk reaction to it.

Asked to respond to the furor, another official with U.S. Central Command said Friday that the organization is trying to remain transparent about U.S. operations in Iraq and provide “more detail and fidelity” about them.

He also noted that the plan to take back Mosul has been discussed for months, including by Iraqi Prime Minister Haider al-Abadi in an interview with the BBC this week. The Iraqi leader told the news organization that he hoped Mosul would be liberated within a few months, but declined to provide more details on when it could occur.

The Centcom official defended the decision to share details Thursday and said the plan is subject to change.

“All of that stuff could be subject to change based on future assessments,” he said. “We are not committed to launching a Mosul offensive on a specific date.”

At the Pentagon, another official said “not one word of operational value” to the Islamic State was released Thursday. Officials appeared to make the case that the information released Thursday has value as a part of psychological operation, in which information is released in an attempt to influence the enemy and civilians in an area of military operations.

“It was all very much big arrow, little map, and highlighted Iraqi commitment to taking back the city and provided some data points to underscore the scope and general timeline of the operation,” the Pentagon official said, speaking on background. “Putting out the date forced [the Islamic State] into a defensive crouch, which saps their energy.”

U.S. military officials have forecast large operations in the past when it suits them. In fall 2004, for example, U.S. commanders made it clear they planned to take back the city of Fallujah in western Iraq from insurgents, allowing those who wished to flee the city to do so. Some 3,500 to 4,000 enemy fighters were in the city when the Second Battle of Fallujah — the bloodiest of the U.S. war in Iraq — began Nov. 7, 2004, said Maj. Gen. Lawrence D. Nicholson in an interview with Checkpoint around the 10th anniversary of the battle.

U.S. Marines also disclosed they were planning to assault the Taliban stronghold of Marja in Afghanistan in January 2010, a few weeks before the battle began. That raised questions at the time whether U.S. commanders were attempting to influence the Taliban with psychological operations.

It isn’t clear what will occur with Mosul. For now, U.S. troops will continue to train Iraqi soldiers for the battle. About 2,000 have graduated already, and another 3,400 are currently in training, the Centcom official said Thursday.

This post was updated with information about the letter McCain and Graham sent the White House.