President Barack Obama has acknowledged that US agencies underestimated the threat posed by the Islamist insurgency in Syria.
In a frank TV interview, he said that al-Qaeda had been beaten in Iraq by US forces working with Sunni tribes.
But they took advantage of the power vacuum in neighbouring Syria to emerge as Isis, later called Islamic State.
Meanwhile, there has been fierce fighting to the west of the Iraqi capital, Baghdad.
The BBC’s Lyse Doucet in the city says Islamist militants were held off by government troops with the help of air strikes by the US-led coalition.
However, the insurgents have not retreated and some are less than 10km (six miles) from the city, she adds.
In an interview with the CBS TV programme 60 Minutes, Mr Obama said Syria had become a “ground zero” for militants who had been able to take advantage of the chaos there.
He reiterated that only part of the solution to defeating them would be military and that a political solution was also necessary.
