Obama speaks at the Global Health Security Agenda Summit in Washington Thomson Reuters 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 according to a clip of the interview broadcast earlier.

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

Obama last week expanded U.S.-led air strikes, which began in Iraq in August, to Syria and he has been seeking to build a wider coalition effort to weaken Islamic State. This 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."

But he said a political solution is necessary in both Iraq and Syria for peace in the long term, according to the interview, which will be broadcast in full on Sunday night.

(Editing by Stephen Powell)