President Obama last night admitted that American intelligence agencies underestimated Isil activity inside Syria, which has become "ground zero" for jihadists worldwide.
In one of his frankest interviews to date Mr Obama said that conversely, the United States overestimated the ability of the Iraqi army to fight the militant groups,.
The president was speaking in a "60 Minutes" interview. Citing earlier comments by James Clapper, director of national intelligence, Mr Obama accepted that US intelligence underestimated what had been taking place in Syria.
Islamic militants went underground when US Marines quashed al Qa'ida 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 swathes of the country that are completely ungoverned, they were able to reconstitute themselves and take advantage of that chaos," Mr Obama said according to a clip of the interview broadcast earlier.
"And so this became ground zero for jihadists around the world."
Mr 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," Mr Clapper was quoted as saying. "I didn't see that. It boils down to predicting the will to fight, which is an imponderable."
Mr 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.
Meanwhile the US undertook more air strikes yesterday on three makeshift oil refineries in Syria's Raqqa province.
The Britain-based Syrian Observatory for Human Rights said the attacks occurred shortly after midnight, adding that they also hit a plastic factory.
Islamic State fighters have control over oil produced in eastern Syria and have set up small, makeshift refineries to distil the crude into fuel, one of their main sources of income.
"These so-called refineries are not a real target and they do not weaken Islamic State as they do not have any financial value for them," Rami Abdelrahman of the Observatory said..
Scores of fighters have left al Qa'ida's Nusra Front and other Islamist groups in Syria to join IS since the strikes started.
The air strikes have failed so far to stop the advance of Isil on Syria's Kurdish town of Kobani near the border with Turkey which the group has sieged from three sides, triggering an exodus of more than 150,000 refugees and sending mortar shells inside Turkish territory.