A gruesome video released online Tuesday showing the beheading of US journalist James Foley by a man who appears to be a British national has sparked international outrage and is fueling fears about the increasing numbers of foreign fighters joining the ranks of the Islamic State.

The Guardian newspaper quoted an unidentified former prisoner held in Syria who said that the executioner goes by the name “John” and is an educated and devout radical Islamist who is one of three UK-born IS militants known as “the Beatles.”

According to the report, the executioner was from London, and was one of 500 British nationals who had traveled to Syria to fight against the regime of Bashar Assad and had been “brutalized” by Islamic State terrorists.

The three “Beatles” are said to be the main guards for foreigners held hostage in Raqqa, a city in northern Syria.

British Prime Minister David Cameron said Tuesday that the man had not been identified but “from what we have seen it looks increasingly likely that he is a British citizen.”

The Guardian quoted sources in Syria saying the executioner had been the main contact for negotiations with families of foreigners held by IS.

On Wednesday, Phil Balboni, the CEO of GlobalPost, the news outlet Foley was photographing for in Syria, said he had received word from IS that Foley would be executed unless their demands were met.

Balboni said GlobalPost had spent millions of dollars to try and locate Foley, but was unable to meet IS ransom demands because of financial restraints and US legal restrictions.

Foley, a 40-year-old American freelance journalist kidnapped in Syria in 2012, was shown in an orange jumpsuit kneeling next to a black-clad militant, with a knife to his throat, delivering what is likely a coerced message condemning US strikes on IS targets in Iraq before he is beheaded.

American journalist James Foley, kneeling in orange, in a video released by the Islamic State that apparently showed him being beheaded by his captor, August 19, 2014. (screen capture: YouTube/News of the World)

American journalist James Foley, kneeling in orange, in a video released by the Islamic State that apparently showed him being beheaded by his captor, August 19, 2014. (screen capture: YouTube/News of the World)

The unidentified Islamic State militant who addressed the camera in the five minute video, is believed by linguists to be from London or Southern England based on his accent, which is described as the “multicultural London English” spoken by many young, inner-city residents from a variety of ethnic backgrounds.

“He sounds to me like a native speaker … or a non-native who has spent a lot of time in London,” said Dominic Watt, a forensic linguist at the University of York.

“They clearly wanted to use a fluent English speaker to ensure the clip was widely used in the US media,” said Peter Neumann, the director of the International Center for the Study of Radicalization at King’s College London. “An American would have been ideal, but there still aren’t many American fighters in the conflict, and it may have been difficult to find one in the place where the hostage was held.”

Syria’s civil war, in its fourth year, has attracted thousands of foreign fighters from around the world. Several hundred people from Britain have traveled to Syria, according to official estimates, and some may have crossed into Iraq as Islamic State militants advanced. France and Germany have estimated a combined 1,300 of their citizens have joined the fight.

Extremists have increasingly used their foreign-born members for propaganda purposes. In June, the Islamic State released a video showing British and Australian militants exhorting compatriots to join them in violent jihad. Last month, an al-Qaeda-linked group in Syria released a video of an American carrying out a suicide attack.

An Islamic State fighter from Australia posted a picture on Twitter showing his 7-year-old son holding the severed head of a Syrian soldier — an image US Secretary of State John Kerry called “one of the most disturbing, stomach-turning, grotesque photographs ever displayed.”

One of the group’s most prominent commanders, appearing frequently in online videos, is Omar al-Shishani, a red-bearded ethnic Chechen.

Nigel Inkster, a terrorism expert at the International Institute for Strategic Studies, said the videos reflect an increasingly sophisticated media strategy designed to energize recruits and give the West a message “of fear and a perception of inevitability.”

He said showcasing large numbers of foreign — and particularly Western — fighters is intended to tell potential recruits that the Islamic State is “a successful movement … and if you want to be a jihadi you have to be part of it.”

Counter-terrorism officials are concerned that Westerners joining the Islamic State in Syria and Iraq will return home and commit acts of terrorism in retaliation for Western attacks against ISIS.

The United States launched additional airstrikes on Islamic State targets in Iraq Wednesday and US senior officials say that that additional troops may be sent to Iraq despite the insurgents’ threat to execute an additional American hostage in retribution for any continued attacks.