Ammon News - AMMONNEWS - Barack Obama sent US troops to Syria this summer on a secret mission to rescue Americans, including beheaded journalist James Foley, held by the Islamic State group but the operation failed, officials said.

Officials told the Associated Press news agency that the rescue mission was authorised after intelligence agencies believed they had identified the location inside Syria where the hostages were being held.

But the several dozen special operations forces dropped by aircraft into Syria did not find them at that location and engaged in a firefight with Islamic State fighters before departing.

"This operation involved air and ground components and was focused on a particular captor network within ISIL [currently known as IS]. Unfortunately, the mission was not successful because the hostages were not present at the targeted location," a Pentagon statement said on Wednesday without specifying which hostages were involved.

A number of the IS fighters, but no Americans, were killed, officials told AP.

The officials would not say specifically when or where the operation took place, citing the need to protect operational details in order to preserve the ability to carry out future rescue missions.

Obama had earlier on Wednesday vowed that the US would not stop its airstrikes against the Islamic State group because it beheaded Foley, an act he said was proof that the fighters stand for no religion.

Obama's response to the killing marked his strongest condemnation yet of the group, calling it a "cancer" with a bankrupt ideology.

His remarks came shortly after the White House announced that a video showing the beheading of Foley had been authenticated by US intelligence, Reuters news agency reported.

"The United States of America will continue to do what we must do to protect our people," Obama said.

"We will be vigilant and we will be relentless. When people harm Americans, anywhere, we do what's necessary to see that justice is done."

Hunt for British killer

Obama said the fighters had rampaged across cities and villages, abducted women and children and subjected them to torture and rape and killed Muslims, both Sunni and Shia, by the thousands.

Earlier, UK Prime Minister David Cameron interrupted his holiday to return to London to hold an emergency meeting about the threat the Islamic State group posed.

The video showing Foley's killing featured a man speaking in what appeared to be a British accent.

Cameron will meet with officials from the Home Office, Foreign Office and intelligence agencies.

"We have not identified the individual responsible on the video but from what we've seen it looks increasingly likely that it is a British citizen. Now this is deeply shocking," Cameron said on Wednesday.

"Our intelligence services will be looking very carefully on both sides of the Atlantic at this video to establish its authenticity, to try to identify the individual concerned and then we will work together to try to locate him," UK Foreign Secretary, Philip Hammond, told British broadcaster Sky news in a reference to the masked man.

Foley, 41, was contributing videos and stories to the AFP news agency for the media company GlobalPost before he was kidnapped on November 22, 2012 by unidentified gunmen.

*Agencies