New US strikes in Iraq after IS Foley video
US aircraft have launched fresh strikes against Islamic State (IS) militants in Iraq, despite threats from the group to kill a second American captive.
US Navy fighters and drones provided air cover for Kurdish and Iraqi forces battling IS near the city of Mosul.
On Tuesday IS released a video showing the beheading of journalist James Foley. In it, the militants threatened to kill another American reporter.
The US attorney general says a criminal investigation will look into his death.
In the UK, police and security services are trying to identify the jihadist who appeared in footage of Mr Foley's killing.
Unconfirmed reports suggest the man - who has an English accent - is from London or south-east England.
In its video of Mr Foley, IS said it was killing him in revenge for US air strikes.
Since the video was released, American forces have conducted 14 new strikes near the strategically important Mosul dam, a key facility recaptured from IS militants earlier this week.
The raids provided air cover as Kurdish and Iraqi government forces pushed into the hills south-west of the site, Kurdish sources said. US officials said they had successfully destroyed 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.
On Wednesday, US President Barack Obama called Mr Foley's 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 had "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.
Foreign fighters in IS
There are estimated to be about 3,000 citizens from Western countries currently fighting for IS in Iraq and Syria, the Royal United Service Institute (Rusi) in London says.
According to Rusi, the majority are believed to be from the UK, Belgium, the Netherlands, Germany, France and the Nordic nations.
The UK government says up to 400 British nationals are fighting alongside militant groups.
But these jihadists are said to be far outnumbered by volunteers from Arab nations such as Tunisia, Morocco and Saudi Arabia.
A June 2014 report from the New York-based consultancy the Soufan Group suggested people from at least 81 different countries had become involved, including citizens from Australia, the US, Canada, Ireland, and Spain.
Sources: Rusi, Soufan Group, BBC Analysis and Research
The statement did not specify whether the operation had intended to rescue Mr Foley, who was kidnapped in Syria in November 2012.
However, senior US officials - speaking on condition of anonymity - confirmed this. They added that the troops had killed several IS militants.
Mr Foley, 40, had reported across the Middle East, working for US publication GlobalPost and other media outlets.
'Warning to Obama'
In the IS video, entitled A Message to America, a man identified as James Foley is dressed in an orange jumpsuit, kneeling in desert-like terrain beside an armed man dressed in black.
He gives a message to his family and links his imminent death to the US bombing campaign in Iraq.
Clearly under duress, he says: "I call on my friends, family and loved ones to rise up against my real killers, the US government, for what will happen to me is only a result of their complacency and criminality."
Then the masked militant delivers a warning to the US president: "Any attempt by you, Obama, to deny the Muslims their rights of living in safety under the Islamic caliphate will result in the bloodshed of your people."
After he speaks, the militant appears to start cutting at his captive's neck before the video fades to black.
Another captive, identified as American journalist Steven Sotloff, is shown at the end, with the warning that his fate depends on President Obama's next move.
Mr Sotloff was abducted in northern Syria a year ago.
- Formed out of al-Qaeda in Iraq (AQI) in 2013, IS captured Raqqa in eastern Syria
- By early 2014 it controlled Falluja in western Iraq
- It has since captured broad swathes of Iraq, seizing Mosul in the north in June and the Mosul dam in August
- The violence has displaced an estimated 1.2 million people in Iraq alone
- Pursuing an extreme form of Sunni Islam, IS has persecuted non-Muslims such as Yazidis and Christians, as well as Shia Muslims, whom it regards as heretics
- In July alone, IS expanded dramatically, recruiting some 6,300 new fighters largely in Raqqa, an activist monitoring group said