Killer of US journalist will be caught says Ex MI6 boss

London - The suspected British killer of US journalist James Foley will be found and brought to justice "sooner or later", Britain's former anti-terrorism chief said on Thursday.

Richard Barrett, former head of counterterrorism at foreign intelligence service MI6, told BBC Radio 4's Today programme that those who knew the suspect would be so appalled by the grisly online video of the killing that they would contact police.

"Ultimately they'll be able to identify him," he said. "He will have had many friends and acquaintances and family in the UK, and those people will wish to see him brought to justice."

The video of Foley's execution was posted online by the Islamic State (IS) jihadist group on Tuesday, prompting global condemnation.

The distinct English accent of the militant who carried out the act has left Britain nervously wondering how many potential jihadists are walking its streets.

Increasingly likely killer a Brit - Cameron

British Prime Minister David Cameron, who broke off his holidays for emergency meetings on the threat posed by IS, called the murder "brutal and barbaric" and said it was "increasingly likely" that the killer was a UK citizen.

Barrett insisted that links between the authorities and the "vulnerable" communities in which it is presumed the suspect lived before heading to Syria to fight with the IS militants were now "very much stronger".

"Nobody is now in any doubt about what it means to go over to Syria or Iraq and join... the IS, so they're very concerned about any members of their community who might do that," he told the BBC.

"So long as they have confidence that by talking to the police they won't bring down all sorts of additional problems to themselves, I think they're ready to do so and I think increasingly they feel they should do so."

'A bit of theatre'

Even when the suspect is identified, the former intelligence official warned it could take a long time before he was brought to justice due to the unrest in Syria, where he is assumed to be.

But given the scale of the crime, and the fact the suspect is assumed to be relatively young, Barrett predicted the attacker would be vulnerable to arrest "sooner or later".

The terror expert called the video "a bit of theatre" designed to strike fear in the hearts of Western civilians, which would, as a result, intensify calls for the United States and its partners to take action against the IS.

"Any sense that you are drawing back could be used by the IS to say 'we've scared you off by doing this' and that's not a good message to send all the many millions of people in Iraq and Syria who are suffering far more directly than the people in the West," he warned.

IS, also known as ISIL, has overrun large swaths of Iraq and Syria, claiming to represent the aspirations of a global Islamic caliphate.