US aircraft have launched fresh strikes against Islamic State (IS) militants in northern Iraq, despite threats from the group to kill asecond American captive in retribution for continued attacks.
US Navy fighters and drones provided air cover to Kurdish and Iraqi forces battling IS near the city of Mosul.
On Tuesday the group released a video showing the beheading of journalist James Foley, who went missing in 2012.
In it, the militants threatened to kill another US reporter they are holding.
But since the video was released, American forces have conducted 14 new strikes near the Mosul dam, a key facility recaptured from IS militants earlier this week.
The raids provided air cover as Kurdish and Iraqi forces pushed into the hills south-west of the site, Kurdish sources said. US officials said they had successfully eliminated vehicles and other targets belonging to IS.
The US has been carrying out strikes against IS - which has been seizing large parts of Syria and Iraq - since 8 August.
In its video, the group said it has killed Foley in revenge for such attacks. US President Barack Obama on Wednesday called the killing "an act of violence that shocks the conscience of the entire world".
Also on Wednesday, the Pentagon said the US had "attempted a rescue operation recently to free a number of American hostages held in Syria".
It said the operation "involved air and ground components".
"Unfortunately, the mission was not successful because the hostages were not present at the targeted location."
It was the first time the US government had acknowledged that its forces had operated in Syria since the country's civil war began in 2011.
The New York Times newspaper quoted unnamed US officials as saying the raid occurred at an oil refinery in the north.
They say commandos were dropped by helicopters, but found that the hostages had recently been moved.