“It’s a three-front war,” a U.S. official said of the FSA rebels’ fight: They face the Assad regime, forces from its Lebanese ally Hezbollah, and now the multinational jihadist ranks of ISIS.

Brigade leaders of the FSA say that ISIS, an Iraqi al Qaeda outfit whose formal name is the Islamic State of Iraq and al-Sham, has dragged them into a battle they are ill-equipped to fight.

Some U.S. officials said they see it as a battle for the FSA’s survival.

In recent months, ISIS has become a magnet for foreign jihadists who view the war in Syria not primarily as a means to overthrow the Assad regime but rather as a historic battleground for a larger Sunni holy war. According to centuries-old Islamic prophecy they espouse, they must establish an Islamic state in Syria as a step to achieving a global one.