Speaking to CBS News, the president said that former Al-Qaeda fighters driven from Iraq by US and local forces had been able to gather in Syria to form the newly dangerous Islamic State group.
A US-led coalition of Arab and Western allies have begun an air campaign to counter the group, hitting targets in Iraq and Syria, which Obama called "ground zero for jihadists around the world."
"I think our head of the intelligence community, Jim Clapper, has acknowledged that they underestimated what had been taking place in Syria," he said, referring to his director of national intelligence.
The comments came in advance excerpts of an interview with the "60 Minutes" news show, due to air in full later today.
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Obama said IS propagandists had become "very savvy" with social media and lured new recruits "who believed in their jihadist nonsense" from Europe, America and Australia, as well as from Muslim countries.
The president said that part of the solution would be military, citing the US-led strikes to deny the IS group territory and resources, but that Syria and Iraq would also have to resolve their political crises.