CIA Director John Brennan says the Islamic State (IS) group in a phenomenon that will "challenge us" for a long time as he marked the five-year anniversary of the killing Al-Qaeda leader Osama bin Laden.
Brennan said on NBC television on May 1 that taking out the head of the IS would make a "great impact" in defeating the extremist Islamic group.
The IS is led by Abu Bakr al-Baghdadi, whose whereabouts are unknown.
"He is important, and we will destroy ISIL, I have no doubt in my mind," said Brennan. "We have to remove the leadership that directs the organization to carry out these horrific attacks."
U.S. special forces killed bin Laden in Pakistan on May 2, 2011.
"We have destroyed a large part of Al-Qaeda [but] it's not completely eliminated," he said. "So we have to stay focused on what it can do."
"Now, with the new phenomenon of [IS], this is going to challenge us for years to come," he said.
He added that the IS group is large and is "not just an organization, it's a phenomenon."