By Mark Hosenball
WASHINGTON (Reuters) - An Internet video surfaced this week depicting a masked man in Syria with a working-class British accent, urging British Muslims to travel there and join one of the most radical Islamist rebel militias fighting to oust President Bashar al-Assad.
In the three-minute video, the man, wearing a black hood with eyeholes, loads what he says is a Glock 19 pistol with bullets he says come from the Free Syrian Army, a more moderate rebel group supported by the West.
"Where are you when they are slaughtering our children and our fathers?," the man says during the course of a diatribe in which he taunts fellow British Muslims and encourages them to go into battle.
After firing a burst of gunfire into the air, the video ends with the man saying: "I also invite you all over to the land of jihad."
The video, posted on YouTube on December 17 and being studied by Western intelligence agencies, highlights what security officials say is an increasingly large role played in the Syrian conflict by foreign fighters, including Europeans. (https://www.youtube.com/watch?v=vXmQPm5-vrw)
U.S. and British officials say the number of British citizens who have traveled to Syria to join anti-government rebels - often the most radical factions - is in the "low hundreds," and that as many as 100 Britons are in Syria fighting with militants at any given time.
Earlier this week, British broadcaster Sky News aired a report featuring interviews with members of what it described as a "previously unknown brigade of British jihadists" fighting with anti-government rebels in northern Syria, near the Turkish border.
UK officials are particularly concerned that Britons who have fought with militants in Syria will return more radicalized, with both new paramilitary skills and with direct contacts to Al Qaeda or its affiliates.
British authorities are believed to be investigating at least a handful of potential terrorism plots involving former foreign fighters in Syria.
The video, which could not be independently authenticated, was posted by a group called Rayat al-Tawheed. The group claims to be the distributor of English-language materials produced by the Islamic State of Iraq and the Levant (ISIL), according to Laith Alkhouri of Flashpoint Partners, which monitors extremist websites for private and U.S. government clients.
ISIL is one of two rebel groups which U.S. and European security officials say are the most extreme Sunni factions seeking to overthrow Assad, who is an Alawite, an offshoot of the Shiite branch of Islam.
A U.S. official said that ISIL is a "subset" of Al Qaeda in Iraq, the Iraqi-based affiliate of what remains of al Qaeda's core organization, but is also a spinoff of al Nusrah, arguably the most violent Sunni militant group fighting Assad.
Alkhouri said the group that posted the latest video had surfaced for the first time this month, and much of the material it has circulated "is on the far radical side, including gruesome images of beheading and killing of Syrian soldiers...
"The group has also posted images of dead 'Shia' fighters, in a slang referring to them as 'tourism photos'," Alkhouri said.
The travels of British citizens and other Europeans to and from Syria are difficult for security agencies to track because there are many routes and limited border controls between nations that belong to the European Union.
Western officials say they fear that Syria is now becoming the central front in Islamic militants' struggle to spread their control and ideology.
(Editing by Warren Strobel and Leslie Gevirtz)