Barack Obama admits United States underestimated Islamic State

US President Barack Obama acknowledged that his intelligence agencies underestimated the threat from Islamic State militants and overestimated the ability and will of Iraq's army to fight.

Questioning Obama's strategy to destroy the group, House of Representatives Speaker John Boehner said the US may have "no choice" but to send in troops if the mix of US-led air strikes and a ground campaign reliant on Iraqi forces, Kurdish fighters and soon-to-be trained Syrian rebels failed to achieve that goal.

Boehner, in an interview broadcast on Sunday, did agree with the White House that Obama had the power to order air strikes in Iraq and Syria, but said he believed Congress should consider a resolution authorising the use of force for this mission.

Boehner said he would bring lawmakers back to Washington if Obama were to seek such a resolution.

Obama described the US intelligence assessments in response to a question during a CBS 60 Minutes interview aired on Sunday night. He was asked about how Islamic State fighters had come to control so much territory in Syria and Iraq and whether it was a surprise to him.

The president said that during the Iraq war, US military forces with the help of Iraq's Sunni tribes were able to quash al-Qaeda fighters, who went "back underground".

"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," Obama said.