US intelligence agencies have underestimated the activity of armed groups inside Syria, which has become "ground zero" for them worldwide, US President Barack Obama has said in an interview aired by American CBS television.

Conversely, the US overestimated the ability of the Iraqi army to fight the groups, Obama said in the interview taped on Friday and aired on Sunday, days after the US president globally made his case for action in a speech at the UN General Assembly in New York.

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

Washington last week expanded the US-led air strikes on the ISIL fighters to Syria, in addition to Iraq [Reuters]

Fighters went underground when US 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."

US and its allies, including Gulf and Western countries, has been conducting air strikes on the self-declared jihadist Islamic State in Iraq and the Levant (ISIL) group, which controls large parts of land and oil fields in Iraq and Syria.

The strikes aim at supporting the Kurdish and central Iraqi forces fighting against the ISIL on the ground.

Obama last week expanded US-led air strikes on the ISIL fighters, which began in Iraq in August, to Syria and he has been seeking to build a wider coalition effort to weaken the group. The ISIL has killed thousands of people and beheaded at least three Westerners.

Clapper told a Washington Post columnist earlier in the month that US intelligence had underestimated the ISIL and overestimated Iraq's army.

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

'Political solution necessary'

In the interview, Obama outlined the military goal against the ISIL: "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 fuelling, and cut off their financing, and work to eliminate the flow of foreign fighters."

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

"I think there is going to be a generational challenge. I do not think that this is something that is going to happen overnight," Obama said, citing an environment in which young men "are more concerned whether they are 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 continued: "What our military operations can do is to just check and roll back these [fighter] 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 added that he recognised the contradiction in opposing the rule of Syrian President Bashar al-Assad while battling ISIL fighters 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."

Source: Agencies