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The new rebel alliance also may signal a rift between the two al Qaeda affiliates fighting in Syria. Jabhat al-Nusrah, also known as the Nusrah Front, has taken a lead role in the alliance, while the Islamic State of Iraq and the Levant has been left out. The Nusrah Front, which the State Department has listed as a terrorist organization, is also one of the most effective rebel militias fighting the Assad regime.

Extremist and more moderate rebel groups fighting Mr. Assad’s forces have increasingly turned on each other in recent months. Last week, fighters from the Islamic State of Iraq and the Levant defeated units of the Free Syrian Army and seized control of Azaz, a strategic town close to the border with Turkey.

Those Islamist rebels have the advantage of a steady supply of arms and money, thought to originate from Saudi Arabia and Qatar. As a result, they have increasingly outperformed the more moderate groups fighting the Assad regime since March 2011 in a war that the United Nations estimates has claimed more than 100,000 lives.

“The secular opposition is being overshadowed, in terms of performance, by these Islamist militias,” said Mr. Schenker. But these Islamist groups “have not, until now, really tried to translate their performance on the ground into a larger political framework.”

“So the Islamists, I think, came to the understanding that ‘Why not try and supplant or at least compete with these secular opposition groups politically?’” he added.

Other members of the new rebel alliance include the Tawheed Brigade, which is thought to be the largest rebel group in the northern city of Aleppo, and the 19th Division, which identifies itself as part of the Free Syrian Army.

“Most of the groups battling against Assad are composed of Islamist fighters, but only a small minority could accurately be characterized as extremist,” a U.S. official said on background.

The radicalization in the opposition’s ranks has been fueled, in part, by the Obama administration’s decision to shelve its plans to take military action against the Assad regime for using chemical weapons, say Western analysts and Syrian activists.

Backlash from Syrians

Many Syrians view the opposition coalition in Turkey as fractious and out of touch with the revolution.

Islamists, too, have faced a backlash from the local population as they have sought to establish control in the northern and eastern parts of Syria. While these groups have provided much-needed services, they have angered locals by seeking to impose Islamic law.

“The balance of power is such that these Islamist militias see themselves as ascendant and also see that their rivals aren’t really getting outside backing like they are,” Mr. Schenker said.

“So it really falls on us to wonder why we are not doing more to strengthen the more moderate opposition. We talk about them periodically, but we are doing precious little to advance their cause vis-a-vis their Islamist rivals,” he said.

President Obama, in a speech at the United Nations on Tuesday, pledged $340 million in additional assistance to Syria and reiterated his support for the moderate opposition. However, U.S. officials, who have to deal with a hodgepodge of rebel groups, are worried about the aid falling into the wrong hands.

A photograph circulating on the Internet purports to show a commander of the al Qaeda-linked group inside a shelter bearing the logo of the U.S. Agency for International Development. Karl Duckworth, a USAID spokesman, said he could not authenticate the photograph.

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