While the U.S. already is providing money, equipment, training and limited weaponry to the Free Syrian Army (FSA), some U.S. military officials are growing increasingly concerned about the presence of extremist Islamic groups within the overall force. A U.S. military strike, even a limited one, could tip the balance in the civil war in favor of the rebels, they argue, potentially propelling these or other radical Islamic groups into a position of power in a post-Assad Syria. …

The FSA is by far the largest Syrian rebel force. U.S. Secretary of State John Kerry and Sen. John McCain have both estimated the total rebel force in Syria at around 100,000. They claim the hardcore Islamists — defined as members of two al Qaeda-affiliated groups waging war against Assad — account for only 10 to 15 percent of that total. …

But a senior military official, also speaking on condition of anonymity, disagreed, claiming the percentage of radical Islamic fighters is “way higher than that,” when the definition is expanded beyond the membership of the al Qaeda affiliates. He said Pentagon officials estimate that extreme Islamist groups now constitute “more than 50 percent” of the rebel force, “and it’s growing by the day.”