The spokesman for American operations against the Islamic State (ISIS/ISIL) in Iraq and Syria declined to predict when the U.S.-backed local troops would retake Mosul, suggesting the final battle to liberate the Iraqi city may be long.

“They will liberate Mosul — it’s just a matter of time when that’s going to happen — but I’m not going to put a timeline on that for them,” American Col. Ryan Dillon, a spokesman for anti-ISIS coalition against ISIS in Iraq and Syria, told reporters in London.

“I am confident they will retake Mosul,” he added. “This is inevitable. It will happen.”

The U.S.-backed Iraqi military and their allies – which include Iran-allied Shiite militiamen and Kurdish fighters, among others – launched the Mosul offensive in October 2016.

Although Iraqi officials foresaw success within a few months, ISIS jihadists have managed to hold onto strategically important parts of western Mosul.

At the beginning of the year, the U.S.-backed forces recaptured the eastern part of the besieged city.

Col. Dillon attributed the slow pace of the ongoing Mosul operation to efforts to minimize civilian casualties as well as ISIS’s determination to use human shields, booby traps, and suicide bombers.

“West Mosul is arguably the most complex and dangerous combat urban environment that any force has seen in decades,” declared the American colonel.

His comments came as the Iraqi forces, supported by American advisers and air power, were preparing to begin what they hope will be the final effort to push ISIS out of Mosul, the second-largest city in Iraq.

CBS News reports:

Over the past couple of days, there has been an uptick in the hostilities between both sides after the Iraqi forces announced a launch of a new offensive to take over the old city. Now we have three Iraqi forces coming from the south, the north and the west putting the pinch on ISIS fighters who remain [in Mosul] — and they are pinned up against the Tigris River.

The Old City itself is densely populated and tightly packed with narrow roads and alleyways that make it difficult if not impossible for Iraqi forces’ armored vehicles or heavy equipment to get into. That means Iraqi forces are going to get to get out and fight this battle on foot. And that gives ISIS something of an advantage.

Only an estimated couple of hundred ISIS jihadists remain in the city, down from about 3,000 to 5,000 at the start of the operation.

There are still between 250,000 and 550,000 civilians in Mosul, said Col. Dillon.

In March, Iraqi Lt. Gen. Talib Shaghati, the chief of Iraq’s elite Counter Terrorism Service (CTS), predicted that Mosul would fall “within a month.”