US President Barack Obama has vowed that his country will not be swayed from airstrikes against the Islamic State (IS) group after it beheaded an American journalist, an act he said was proof that the fighters stand for no religion.

Obama's response on Wednesday to the execution of James Foley marked his strongest condemnation yet of IS fighters, and he gave no sign of a pause in US targeting of its positions in Iraq.

The president's remarks to reporters covering his vacation in Martha's Vineyard, Massachusetts, came shortly after the White House announced that a video showing the beheading of Foley had been authenticated by the US intelligence community, the Reuters news agency reported.

Obama said he had called Foley's family to express his condolences.

"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."

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.

"No just God would stand for what they did yesterday and what they do every single day," he said. "Their ideology is bankrupt."

US Secretary of State John Kerry later called the Islamic State group an "Ugly, savage, inexplicable, nihilistic, and valueless evil."

"ISIL is the face of that evil, a threat to people who want to live in peace, and an ugly insult to the peaceful religion they violate every day with their barbarity," he added.

Cameron meeting

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 to the AFP news agency for the media company GlobalPost before he was kidnapped on November 22, 2012 by unidentified gunmen.