David Ignatius writes: Syrian opposition leaders report an alarming growth within their ranks of fighters from Jabhat al-Nusra, an extremist group linked to al-Qaeda.

The Jabhat group now has somewhere between 6,000 and 10,000 fighters, according to officials of an non-governmental organization that represents the more moderate wing of the Free Syrian Army (FSA). They say that the al-Qaeda affiliate now accounts for 7.5 percent to 9 percent of the Free Syrian Army’s total fighters, up sharply from an estimated 3 percent three months ago and 1 percent at the beginning of the year.

The extremist group is growing in part because it has been the most aggressive and successful arm of the rebel force. “From the reports we get from the doctors, most of the injured and dead FSA are Jabhat al-Nusra, due to their courage and [the fact they are] always at the front line,” said a message sent today to the State Department by the moderate Free Syrian Army representatives, warning of the extremists’ rise.

These estimates are very rough, given the scattered and disorganized nature of the opposition. But they are based on detailed reporting from the field by the members’ military councils, which are the closest thing to an organized command structure among the rebels. [Continue reading…]