The U.S.-led coalition fighting against the Islamic State has prepared “all the pieces” it needs to take back the Iraqi city of Mosul, and expects military operations there are “rapidly approaching,” a senior State Department official said Friday.

Brett McGurk, the State Department’s envoy to the coalition, said the mission to take back the city of more than 1 million people will be “a very unpredictable, very dynamic, very uncertain operation.” It will be the most ambitious and complex operation the Iraqi government has undertaken against the Islamic State since it roared across the country in 2014, and include approximately 30,000 Iraqi forces.

McGurk, speaking at the State Department, said there’s a “humanitarian imperative” to expel the militants from Mosul as soon as possible. But he acknowledged putting the city back together will be a major undertaking. An extensive amount of planning has gone into understanding the political dynamics between different forces involved in the battle, the humanitarian assistance that will be required, and how local government will be organized after the battle, he said.

The remarks come after weeks of some U.S. and Iraqi officials questioning whether enough planning has been done to launch the battle, particularly in light of long-simmering political and religious tensions between groups that will be allied against the Islamic State in the operation. Concerns also remain that fighting in Mosul will provoke a sprawling humanitarian crisis with up to one million people internally displaced in Iraq.

McGurk acknowledged the concerns Friday, saying that while not all details of a transition after the battle have been finalized, the approach in place has worked “organically” in other Iraqi cities retaken from the Islamic State. He called the estimate that one million people will be displaced “apocalyptic” and unlikely to occur, and said land, supplies and accommodations will be ready for up to 750,000 people.

“If we try to resolve everything before Mosul, Daesh will never get out of Mosul,” McGurk said, using an alternate name for the militants. “And this is really a war of momentum. We feel that momentum is on the side of the Iraqi security forces. But they are the ones that will set the date for when this launches.”

A U.S. military spokesman in Baghdad, Air Force Col. John Dorrian, agreed with McGurk’s assessment on when the operation could begin. Defense Secretary Ashton B. Carter authorized the deployment of an additional 600 U.S. troops to assist Iraqi troops in the Mosul operation late last month, and “a lot of them” have arrived already, Dorrian said.

“We’re at the point where the Iraqis will initiate this at the time of their choosing,” Dorrian said. “At this point, we’re just waiting for the Iraqis to make the determination on when they’re ready to go.”

U.S. military advisers are in the process of training the last of 12 Iraqi army brigades that could be involved in the offensive, Dorrian said. Each of them include between 800 and 1,600 soldiers. Many of the U.S. advisers are based at Qayyarah Airfield West, a base that Iraqi forces seized in July and about 40 miles south of the city. It will provide a landing spot for both helicopters and cargo planes during the offensive, serving as a key logistics hub.

An Obama administration official, speaking on condition of anonymity after McGurk’s remarks, said the envoy’s pronouncement should be considered significant. It’s unlikely the offensive will include the kind of airpower that was used during the “shock and awe” campaign at the outset of the U.S. invasion of Iraq in 2003, the official said, but airstrikes will be carried out to allow Iraqi units to advance.

Karen DeYoung contributed to this report.