WASHINGTON (Reuters) - U.S. intelligence agencies underestimated Islamic State activity inside Syria, which has become "ground zero" for jihadists worldwide, President Barack Obama said in a CBS television interview broadcast on Sunday.

Conversely, the United States overestimated the ability of the Iraqi army to fight the militant groups, Obama said in a "60 Minutes" interview taped on Friday, days after the U.S. president made his case at the United Nations for action.

Citing earlier comments by James Clapper, director of national intelligence, Obama acknowledged that U.S. intelligence underestimated what had been taking place in Syria.

Islamic militants went underground when U.S. Marines quashed al Qaeda in Iraq with help from Iraq's tribes, he said.

"But over the past couple of years, during the chaos of the Syrian civil war, where essentially you have huge swaths of the country that are completely ungoverned, they were able to reconstitute themselves and take advantage of that chaos," Obama said.

"And so this became ground zero for jihadists around the world."

Obama last week expanded U.S.-led airstrikes, which began in Iraq in August, to Syria and he has been seeking to build a wider coalition effort to weaken Islamic State. The group has killed thousands and beheaded at least three Westerners while seizing parts of Syria and northwestern Iraq.

Clapper told a Washington Post columnist this month that U.S. intelligence had underestimated Islamic State and overestimated Iraq's army.

"I didn't see the collapse of the Iraqi security force in the north coming," Clapper was quoted as saying. "I didn’t see that. It boils down to predicting the will to fight, which is an imponderable."

Obama outlined the military goal against Islamic State: "We just have to push them back, and shrink their space, and go after their command and control, and their capacity, and their weapons, and their fueling, and cut off their financing, and work to eliminate the flow of foreign fighters."

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'GENERATIONAL CHALLENGE'

But Obama said a political solution was necessary in both Iraq and Syria for peace in the long term.

"I think there's going to be a generational challenge. I don't think that this is something that's going to happen overnight," Obama said, citing 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 a good job.

Saying a solution involved "how these countries teach their youth," Obama said: "What our military operations can do is to just check and roll back these (militant) networks as they appear and make sure that the time and space is provided for a new way of doing things to begin to take root."

Obama said he recognized the contradiction in opposing the rule of Syrian President Bashar al-Assad while battling Islamic State militants who have been fighting Assad's government.

"For Syria to remain unified, it is not possible that Assad presides over that entire process," Obama said. "On the other hand, in terms of immediate threats to the United States, ISIL, Khorasan Group, those folks could kill Americans."

ISIL is the acronym the U.S. government uses to refer to the Islamic State.

Asked about how despite assembling a large international coalition against Islamic State, it appeared the United States was doing most of the work, Obama replied: "That's always the case.

"America leads," he said. "We have capacity no one else has. Our military is the best in the history of the world. And when trouble comes up anywhere in the world, they don't call Beijing. They don't call Moscow. They call us."

(Reporting by Doina Chiacu and Peter Cooney; Editing by Stephen Powell and Eric Walsh)