Secretary of State John Kerry insisted that the United States' allies in the Middle East remain on course to defeat ISIS - though he acknowledged the challenge in Syria is a particularly difficult one

"Each time the Iraqis have now gone forward in an offensive effort, together with allies, the coalition, we have routed ISIL," Kerry said, using an alternate name for the group, during an appearance on ABC's "This Week" on Sunday.

He said opponents of the group have regained "a fixed, significant percentage” of the ground that the group had taken.

'Now Syria is a different matter. Syria is a bigger challenge. We will need people on the ground, ” Kerry said.

He said US forces won't be on the ground there, but "there's training and equipping of the opposition that begins this month," and Arab countries in the region are stepping up their efforts. Kerry also said he's meeting with Saudi Arabia's King Salman soon.

"There's a great deal of energy and effort being put into this and 1 am confident that over time, we will beat, we will, indeed, degrade and ultimately destroy ISIL, yes," he said.