Story highlights President Barack Obama said the U.S. strategy against ISIS has succeeded in containing them, but has not yet hit their leadership

U.S.-backed forces in Iraq succeeded Friday in liberating a northern Iraqi city from ISIS control

Washington (CNN) President Barack Obama said that the U.S. strategy against ISIS has "contained them," but not yet succeeded in its effort to "decapitate" ISIS leadership.

"I don't think they're gaining strength," Obama told ABC's George Stephanopoulos on Thursday for an interview that aired Friday morning. "What is true, from the start our goal has been first to contain and we have contained them. They have not gained ground in Iraq and in Syria. They'll come in. They'll leave. But you don't see this systemic march by ISIL across the terrain. What we have not yet been able to do is to completely decapitate their command and control structures. We've made some progress in trying to reduce the flow of foreign fighters."

Obama has been apprehensive about engaging U.S. ground forces in another struggle in the Middle East, after extensive wars in Afghanistan and Iraq. But he announced two weeks ago that a small group of less than 50 U.S. special ops soldiers would go to Syria to bolster rebel forces there in the fight against ISIS.

Meanwhile, Kurdish Peshmerga fighters launched an attack, with U.S. support, Thursday on a key ISIS supply line in Sinjar, Iraq. Coalition forces declared the Iraqi city free of ISIS control Friday morning.