Washington is considering direct talks with Iran on the security situation in Iraq, a US official has told the BBC.
The move comes as US President Barack Obama weighs up options on action to take in Iraq.
Meanwhile the US condemned as "horrifying" photos posted online by Sunni militants that appear to show fighters massacring Iraqi soldiers.
In the scenes, the soldiers are shown being led away and lying in trenches before and after their "execution".
The Iraqi military said the pictures were real but their authenticity has not been independently confirmed.
The BBC's Jim Muir, in northern Iraq, says if the photographs are genuine, it would be by far the biggest single atrocity since the time of the American-led invasion of Iraq in 2003.
Their emergence came as the Iraqi government claimed to have "regained the initiative" against an offensive by Sunni rebels led by ISIS - the Islamic State in Iraq and the Levant.
Extremists captured key cities, including Mosul and Tikrit, last week, but several towns have now been retaken from the rebels.
Read more: http://www.bbc.co.uk/news/world-middle-east-27863870