Speaking to CBS News, the president said 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 has 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," Obama said, referring to his director of national intelligence.
Asked whether Washington has also overestimated the ability or will of Iraq's US-trained military to fight the jihadists on its own, Obama said: "That's true. That's absolutely true."
He made his comments in an interview with 60 Minutes taped on Friday.
Obama said IS propagandists had become "very savvy" in their use of social media, and had lured new recruits "who believed in their jihadist nonsense" from Europe, America and Australia, as well as from Muslim majority countries.
The US president said that part of the solution would be for Syria and Iraq to resolve their domestic political crises.
An enduring solution, Obama said, requires "a change in how not just Iraq, but countries like Syria and some of the other countries in the region, think about what political accommodation means".
Some countries in the region, the president said, "have now created an environment in which young men are more concerned whether they're Shia or Sunni, rather than whether they are getting a good education or whether they are able to, you know, have a good
job".
Obama added that US-led air strikes in Iraq are helping deny the IS group vital territory and resources.
"We are assisting Iraq in a very real battle that's taking place on their soil, with their troops," Obama said.
"It is in our interest to do that, because ISIL represents sort of a hybrid of not just the terrorist network, but one with territorial ambitions, and some of the strategy and tactics of an army.
"This is not America against ISIL. This is America leading the international community to assist a country with whom we have a security partnership with, to make sure that they are able to take care of their business."
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