Rebels from al-Qaida affiliated Jabhat al-Nusra sit on a truck full of ammunition after capturing an air base in Idlib province in northern Syria. (Anonymous/AP)

American hopes of winning more influence over Syria’s fractious rebel movement faded Wednesday after 11 of the biggest armed factions repudiated the Western-backed opposition coalition and announced the formation of a new alliance dedicated to creating an Islamic state.

The al-Qaeda-affiliated Jabhat al-Nusra, designated a terrorist organization by the United States, is the lead signatory of the new group, which will further complicate fledgling U.S. efforts to provide lethal aid to “moderate” rebels fighting to topple Syrian President Bashar al-Assad.

Others include the Tawheed Brigade, the biggest Free Syrian Army unit in the northern city of Aleppo; Liwa al-Islam, the largest rebel group in the capital, Damascus; and Ahrar al-Sham, the most successful nationwide franchise of mostly Syrian Salafist fighters. Collectively, the new front, which does not yet have a formal name but has been dubbed by its members the “Islamist Alliance,” claims to represent 75 percent of the rebels fighting to topple Assad.

Gen. Salim Idriss, the head of the moderate Supreme Military Council and the chief conduit for U.S. aid to the rebels, cut short a visit to Paris after the announcement of the alliance overnight Tuesday and will head to Syria on Thursday to attempt to persuade the factions to reconsider, according to the council’s spokesman, Louay al-Mokdad.

The new alliance stressed that it was not abandoning Idriss’s council, only the exiled political opposition coalition, which, it said in a statement, “does not represent us.”

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The creation of the bloc nonetheless leaves Idriss’s council directly responsible for just a handful of small units, calling into question the utility of extending aid to “moderate” rebels, according to Charles Lister of the London-based defense consultancy IHS Jane’s.

If the development holds, he said, “it will likely prove the most significant turning point in the evolution of Syria’s anti-government insurgency to date.”

“The scope for Western influence over the Syrian opposition has now been diminished considerably,” he added.

Mokdad acknowledged that by aligning themselves with Jabhat al-Nusra, the other rebel factions could jeopardize hopes of receiving outside military help, just as the Obama administration says it is starting to step up its support after more than a year of hesitation.

But, he said, the United States and its allies are to blame, for failing repeatedly to deliver on promises to provide assistance as the death toll in Syria, now well over 100,000, steadily mounted.

The development appeared to take the Obama administration by surprise. A senior State Department official, briefing reporters Tuesday night on a meeting at the United Nations between Secretary of State John F. Kerry and Syrian Opposition Coalition Chairman Ahmad al-Jarba, was unaware of the rebel announcement that had been made several hours earlier.

In a statement Wednesday, State Department spokeswoman Jen Psaki said that officials had “seen the reports” and were “discussing with the moderate opposition what impact this will have going forward.”

“A divided opposition benefits the Assad regime and opportunists who are using the conflict to further their own extreme agenda,” Psaki said. U.S. aid would continue, she said, “taking into account that alliances and associations often change on the ground based on resources and needs of the moment.”

At a time when the United States and Russia are accelerating efforts to hold a peace conference in Geneva that would bring together the government and the opposition, the defection of some of the most significant rebel factions comes as a reminder that any negotiated settlement will also have to take into account the wishes of those who wield power on the ground, said Amr al-Azm, a history professor at Shawnee State University in Ohio who is Syrian and supports the opposition.

Azm said he does not believe the mainstream factions have embraced Jabhat al-Nusra’s extremist ideology. Rather, he said, “the brigades on the ground are sending a major shot across the bows of the coalition before talks in Geneva. They are essentially reminding them that they will need to include them.”

“This is why you have a lot of moderates and extremists banding together, because otherwise it’s not a natural fit,” he added.

Mokdad said that Idriss had called some of the rebel leaders Wednesday, “and they told us they signed this because they lost all hope in the international community.”

“They said: ‘We are really tired, Bashar al-Assad is killing us, all the West is betraying us, and they want to negotiate with the regime over our blood.’ ”

Abu Hassan, a spokesman for the Tawheed Brigade in Aleppo, echoed those sentiments, citing rebel disappointment with the Obama administration’s failure to go ahead with threatened airstrikes to punish Assad for using chemical weapons in the suburbs of Damascus last month, as well as its decision to strike a deal with Russia over ways to negotiate a solution.

“Jabhat al-Nusra is a Syrian military formation that fought the regime and played an active role in liberating many locations,” he said. “So we don’t care about the stand of those who don’t care about our interests.”

The statement issued by the rebel groups and read on video by a Tawheed commander attributed their decision mostly to dissatisfaction with the Syrian Opposition Coalition, made up of exiled politicians who have struggled to win support among Syrians in the country even as they have courted the international community.

But the Islamist alliance’s creation also coincides with growing concerns among mainstream rebels about the rapid ascent of the other main al-Qaeda affiliate, the Islamic State of Iraq and the Levant, which expanded into Syria from Iraq earlier this year and has recently taken up arms against other rebel factions to extend its control over rebel territory.

Although the statement did not mention the Islamic State, Abu Hassan acknowledged that the new alliance is also intended to confront “anyone who might harm the security and safety of the Syrian citizen in the liberated areas, and solve differences using weapons instead of going to court.”

He was referring to the recent battles in which the Islamic State wrested control of the northern Aleppo provincial town of Azaz from the local rebel faction, the Northern Storm brigade. There have been other clashes, too, in recent weeks, in the eastern cities of Raqqah, Deir el-Zour and several smaller towns where the Islamic State has succeeded in trouncing more moderate groups.

Jabhat al-Nusra, which was established by a Syrian commander from the Islamic State in 2011, regards itself as less extreme even as it acknowledges its allegiance to al-Qaeda leader Ayman al-Zawahiri. It has also become one of the chief victims of the Islamic State’s expansion.

Although its leader, Abu Mohammed al-Jolani, has resisted the Islamic State’s efforts to merge with Jabhat al-Nusra, many fighters left him to join the Iraqi group, and his fighters have been forced to withdraw from a number of towns they had controlled.

In recent weeks, the group has been quietly recruiting support for an effort to confront the Islamic State, according to two rebel fighters in northern Syria.

Ahmad Ramadan in Beirut and Karen DeYoung in Washington contributed to this report.