Washington ? Syrian opposition leaders report an alarming growth within their ranks of fighters from Jabhat al-Nusra, an extremist group linked to al-Qaida.

Jabhat al-Nusra now has somewhere between 6,000 and 10,000 fighters, according to officials of a non-governmental organization that represents the more moderate wing of the Free Syrian Army (FSA). They say that the al-Qaida affiliate now accounts for 7.5 percent to 9 percent of the FSA’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 Friday 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 FSA’s military councils, which are the closest thing to an organized command structure among the rebels. In reports sent this week to the State Department, the NGO representing the Syrian moderates offered a detailed breakdown of the extremists’ growth:

l In Aleppo, the Jabhat al-Nusra force is reckoned at around 2,000, mostly in the al-Bab area northeast of the city. This estimate is based partly on reports from a doctor in the area who has treated injured fighters. The total FSA presence in the Aleppo area is about 15,000.

l In Idlib province, west of Aleppo, Jabhat al-Nusra’s ranks number 2,500 to 3,000, or about 10 percent of the FSA fighters there.

l In Deir al-Zor, to the northeast, the extremist group has about 2,000 of the FSA’s total force of 17,000, according to the reports. Among Jabhat al-Nusra’s most spectacular operations there were the recent seizures of the Al-Ward oil field and a Conoco gas field, the reports said.

l In Damascus, the Jabhat al-Nusra force is somewhere between 750 and 1,000. Another 1,000 fighters are spread around the country in Latakia in northwest Syria, Homs in the center and Daraa in the south.

The Syrian reports paint a picture of a disorganized rebel force in which the extremists are filling the vacuum caused by the lack of clearly established command and control.

In the chaos of the Syrian battlefield, smaller battalions drawn from neighborhoods or small towns are combining forces with larger groups to form brigades, many of them led by extremists. “This means more (mergers) of extreme groups within Jabhat al-Nusra as it becomes more and more franchised,” the report explains. “Their risk is paying off. They are on a high (rate) of growth.”

A message sent earlier this week from the Free Syrian Army representatives touted the new use of anti-aircraft missiles to down a Syrian helicopter. “It’s thrilling to see it (the anti-aircraft weapon) in action finally. The bad news is that it was not through the U.S. but from the regime bases fallen into the hands of the (FSA) battalions. The other bad news is that it’s not under the control or the supervision of the MC [military council] commanders.”

As the rebels gain momentum, the spoils of war apparently are going to the rebel group that captures a particular Syrian army base. This is one factor boosting the rapid growth of Jabhat al-Nusra, whose fighters are so dedicated. They provide the muscle and weapons and, as a result, explained an official of the NGO that represents the moderate FSA fighters: “They will get all goodies, reputation and recognition.”