Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey said today that the fight against ISIS would remain a "very challenging task" until the Iraqi government is able to win over the substantial Sunni population living between the capital cities of Iraq and Syria.

"The government of Iraq, which is moving but has not yet achieved a narrative that would cause the 20 million Sunnis who live between Damascus and Baghdad to believe that their future is with the government of Iraq, in the case of Iraqis, and certainly the Syrian regime is not reaching out to the Sunni population in Syria," Dempsey told ABC News' Martha Raddatz for "This Week."

"Until those facts change, this is going to be a very challenging task. In other words, until ISIL [ISIS] doesn't have, you know, freedom of movement in and among the populations of Al Anbar Province and Nineveh Province, and in Eastern Syria, this is going to be a challenge," Dempsey said of ISIS, the extremist Islamist group also referred to as ISIL or the Islamic State.

During the interview for "This Week," the general discussed an incident this week when ISIS fighters were within 20 to 25 kilometers of the strategically important Baghdad airport, where Apache helicopters were called in to assist Iraqi forces.

"Had they overrun the Iraqi unit, it was a straight shot to the airport. So we're not going to allow that to happen. We need that airport," Dempsey said.

Dempsey also highlighted the difficulty of targeting ISIS forces as they make efforts to conceal their presence, including blending into local population centers.

"The enemy adapts and they will be harder to target," Dempsey said. "They know how to maneuver and how to use populations and concealment. So when we get a target, we'll take it."

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