Iraq and Syria have been so thoroughly damaged by warfare, sectarian conflict and killing that it is unclear they "can be put back together again," CIA Director John Brennan said.

In an interview this week with the CTC Sentinel, a publication from the West Point military academy's Combating Terrorism Center, the head of the Central Intelligence Agency said the current system of governance in the two countries might change altogether.

"I don't know whether or not Syria and Iraq can be put back together again. There's been so much bloodletting, so much destruction, so many continued, seething tensions and sectarian divisions," Brennan said.

"I question whether we will see, in my lifetime, the creation of a central government in both of those countries that's going to have the ability to govern fairly."

He added that he could envision some type of a federal structure governing autonomous regions.

In northern Iraq and parts of Syria, for instance, Kurdish populations already have established de-facto states.

Brennan also described how the Islamic State group (IS) is now collaborating in Yemen with rivals Al-Qaeda to fight common enemies, such as the Houthi rebels and Arab coalition-backed government forces.

"The farther away you get from that (IS) heartland of Syria and Iraq, the more likely you're going to see collaboration between Al-Qaeda elements, (IS) elements, and others," he said.

"We see it right now in Yemen.... There are indications that, in fact, they're working together."