"I really want to cut your head off right now," he added, telling his men, many of whom appeared to have North African accents, that this American kills Muslims.
With the intervention of nearby villagers, the confrontation eventually was defused. But it underscored the unpredictable element that foreign fighters bring to the Syrian conflict.
Most of those fighting the regime of President Bashar al-Assad are ordinary Syrians and soldiers who have defected, having become fed up with the authoritarian government, analysts say. But increasingly, foreign fighters and those adhering to an extremist Islamist ideology are turning up on the front lines. The rebels are trying to play down their influence for fear of alienating Western support, but as the 18-month-old fight grinds on, the influence of these extremists is set to grow.
On Monday, a U.N. panel reported a rise in the number of foreign fighters in the conflict and warned that it could radicalize the rebellion.
The Syrian government has always blamed the uprising on foreign terrorists, despite months of peaceful protests by ordinary citizens that only turned violent after repeated attacks by security forces. The transformation of the conflict into an open war has given an opening to the foreign fighters and extremists.
Talk about the role of foreign jihadists in the Syrian civil war began in earnest, however, with the rise in suicide bombings. U.S. National Director of Intelligence James Clapper said in February that those attacks "bore the earmarks" of the jihadists in neighboring Iraq.
Rebel commanders are quick to dismiss the role of the foreign fighters and religious extremists, describing their numbers as few and their contribution as paltry.
Col. Abdel-Jabbar Aqidi, a top rebel commander for the Aleppo area, said that there were maybe 500 jihadis involved in the battle for Aleppo, while a report from the Quilliam Foundation, a London-based think tank studying extremism, estimated a total of 1,200 to 1,500 foreign fighters in all of Syria.
Other commanders estimated that at most, jihadis, whether local or foreign, made up no more than 10 percent of the fighters.
While this is a small amount compared with the thousands of rebels estimated to be battling the regime, Peter Harling of the International Crisis Group warns that the religious extremists will have an influence on the rebellion.
"I think numbers are irrelevant," he said, adding that the extremists are a "very important phenomenon in many ways. Their presence is very divisive, whether there are many or not."