The military campaign to take the Iraqi city of Ramadi back from the Islamic State is now two weeks old and going “in many ways as planned,” a senior U.S. military officer said Friday. But Iraqi soldiers are just now at the edge of the city, and the operation remains difficult.

Marine Brig. Gen. Kevin Killea, the chief of staff of Operation Inherent Resolve, told reporters at the Pentagon that Islamic State militants had deployed “complex obstacles” and belts of improvised explosive devices around the city in order to slow the Iraqi advance. Iraqi soldiers are “in the isolation phase,” with the more difficult clearing phase next and certain to test their mettle, the general said.

Breaking an operation into phases is standard for military planning. The initial phase — known as the shaping phase — began July 12, Killea said. The U.S.-led coalition supporting the Iraqi government has unleashed more than 100 airstrikes since. The second phase, now underway, is in many ways an extension of the shaping phase and has involved Iraqi forces cutting off supply lines and escape routes around the city.

Killea did not to go into specifics about the Islamic State’s defenses within the city, but suggested the militants have had ample time to dig in and prepare for the coming Iraqi assault.

“Coalition forces are providing continuous support to the Iraqi Security Forces during the phases of this operation,” Killea said.

The campaign is a key test for the Iraqi security forces, which abandoned Ramadi, the provincial capital of Anbar province, to the militants in May. The city of more than 400,000 people is larger than Tikrit, the last city that was wrested from Islamic State fighters by Iraqi forces and Shiite militias. Sunni tribal fighters also have taken a significant role in fighting the militants outside of Ramadi, making it more difficult to determine who is friend and foe from the air.

The evolving nature of the Iraqi military’s focus has prompted U.S. commanders to rethink how they complement them. In one example, a 26-man Marine Corps advise and assist team deployed to Al Asad Air Base in Anbar province in January, and had expected to assist the Iraqi military in preparing for an assault this summer on the militant-held city of Mosul, said Marine Col. John E. McDonough, its commanding officer.

McDonough said in a recent phone interview that he was eventually sent to Baghdad, and was advising the Iraqi military in Anbar remotely when Ramadi fell in May. McDonough’s team,which recently returned to the United States, included operations planners, intelligence planners, joint terminal air controllers, artillerymen and logistics planners from II Marine Expeditionary Force, of Camp Lejeune, N.C. The Marines stressed the limits of their mission in conversations with Iraqi commanders: no direct combat, but synchronizing operations, intelligence and fire support.

“Though we’re very clear about the things we will help them do, we’re also very clear about what we won’t help them do this time,” McDonough said.

Airstrikes have been a staple of the coalition’s campaign since it began last year, but in an environment that heavily restricts coalition forces from operating on the ground, the process to call in the strikes is somewhat unconventional.

Traditionally, forward observers known as joint terminal air controllers (JTACs) would direct air support from the ground, ensuring that pilots knew where they are dropping bombs in relation to friendly forces. Now, however, the request is received from troops on the ground and sent to a corresponding command center that then dictates the response.

In Iraq and Syria there are two different types of airstrikes: deliberate and dynamic. In a statement, officials with U.S. Central Command said that deliberate strikes have a longer preparation time and are approved by both coalition higher-ups and the Iraqi government. Dynamic strikes, however are “delegated to the appropriate target engagement authority.”

CENTCOM said that the coalition “coordinates and controls all of its airstrikes, we do not direct strikes from the ground.”

Without any feedback from a coalition service member on the ground, targeting is most likely assisted by unmanned aerial vehicles and sensors located within the aircraft themselves.

The use of JTACs in the fight against the Islamic State has been a topic of contention for various lawmakers. Last week, Sen. John McCain (R-Ariz.) grilled Lt. Gen. Robert B. Neller on whether JTACs are necessary in Iraq during Neller’s confirmation hearing to become the next Marine Corps commandant.

“Forward controllers make the difference. This line about (Iraqis forces) … have to do it for themselves – General, they can’t do it themselves. We know that. That’s why they’re losing,” McCain said.

McDonough, however, defended the process used to clear airstrikes in Iraq right now, saying the coalition can take out militants easily despite there being a deliberate and systemic process to clear fires.

“We can kill them pretty good,” he said. “We’ve got this thing about as tight as we can make it right now. It needs to be that way.”