Militants from the Islamic State in Iraq and the Levant, an al-Qaeda affiliate, had detained the two former army officers as they accompanied a weapons shipment, according to Rami Abdurrahman, head of U.K.-based group Syrian Observatory for Human Rights. Mediation efforts over three days failed to secure their release, Abdurrahman said by phone yesterday.

More than two years into Syria’s civil war, radical Sunni Islamists are emerging as the prevalent force seeking to topple President Bashar al-Assad, according to military analysts in Europe and the Middle East. Their influence is among the biggest challenges facing the U.S. and allies such as Saudi Arabia as they decide which anti-Assad forces to back and how.

“Two of the most powerful insurgent factions in Syria are al-Qaeda factions,” Evan Kohlmann, senior partner at Flashpoint Partners in New York, said by telephone. “Even were the Assad regime to fall and there be some kind of takeover by rebels, there’s not a clear understanding that everyone here will be able to agree and form any kind of government.”