The U.S.-led coalition fighting Islamic State (ISIS) needs to take back the cities Mosul and Raqqa and will use "boots on the ground" as part of its strategy in doing so, U.S. Defense Secretary Ash Carter said on Friday.

"We need to destroy them in those two places, and I'd like to get on with that as soon as possible," Carter, speaking from Davos, Switzerland, said in an interview on CNBC.

He said the coalition is using raids and bombs to take control of the routes between the two cities and cut off communication between them.

"That'll essentially separate the Iraqi theater from the Syrian theater," he said.

Carter said more ground soldiers will probably be added to support those already there, but part of the strategy is also mobilizing local forces "rather than trying to substitute for them."