BEIRUT, Lebanon — As diplomats at the United Nations push for a peace conference to end Syria's civil war, a collection of some of the country's most powerful rebel groups publicly abandoned the opposition's political leaders, casting their lot with an affiliate of al-Qaida.
As support for the Western-backed leadership fell away, a second, more extreme al-Qaida group carved out footholds across parts of Syria, frequently clashing with mainline rebels who accuse it of making the establishment of an Islamic state a priority over the fight to topple President Bashar Assad.
The fractured nature of the opposition, the rising radical Islamist character of the rebel fighters and the increasing complexity of Syria's battle lines have left the exile leadership with diminished clout inside the country and have raised the question of whether it could hold up its end of any agreement reached to end the war.
The deep differences between many of those fighting in Syria and the political leaders who have represented the opposition abroad spilled into the open late Tuesday, when 11 rebel groups declared that the opposition could be represented only by people who have "lived their troubles and shared in what they have sacrificed."
Distancing themselves from the exile opposition's call for a democratic, civil government to replace Assad, they called on all military and civilian groups in Syria to "unify in a clear Islamic frame." Those who signed included three groups aligned with the Western-backed opposition's supreme military council.
The statement was issued just as Western nations are striving to raise the profile of the "moderate" Syrian political opposition, which is led by Ahmad al-Jarba. The United States and its allies have been reluctant to fully align with and arm the rebels because they are heavily populated by Islamists.
France has scheduled an event today on the sidelines of the U.N. General Assembly at which Jarba is to speak along with foreign ministers who have backed him, including Secretary of State John Kerry.
There was no immediate comment from Jarba, whose coalition is formally known as the National Coalition of Syrian Revolutionary and Opposition Forces. Jarba canceled a news conference that had also been scheduled for today.
The latest split in the opposition emerged as the United States, Russia and other permanent members of the U.N. Security Council were making progress on another front: drafting a council resolution that would enforce an agreement on eliminating Syria's vast chemical weapons arsenal.
"At this stage, the political opposition does not have the credibility with or the leverage over the armed groups on the ground to enforce an agreement that the armed groups reject," said Noah Bonsey, who studies the Syrian opposition for the International Crisis Group.
"You need two parties for an agreement, and there is no viable political alternative to the coalition," he said, defining a disconnect between the diplomatic efforts taking shape in New York and the reality across the war-torn country.
Inside Syria, rebel groups that originally formed to respond to crackdowns by Assad's forces on political protests have gradually merged into larger groupings, some commanded and staffed by those espousing radical Islamist ideologies. But differences in ideology and competition for scarce foreign support have made it hard for them to unite under an effective, single command.
Seeking to build a moderate front against Assad, Western nations encouraged the formation of the opposition political coalition. Even though some of its leading members like Jarba have been imprisoned by the Assad government, the coalition has loose links to many of the rebel fighters on the ground.
The rebel groups that assailed the political opposition are themselves diverse and include a number that are linked to the coalition's supreme military council. But more troubling to the West, those same groups signed with the Al Nusra front, an al-Qaida-linked group that the United States has designated a terrorist organization.
Further complicating the picture is the rise of the new al-Qaida franchise, the Islamic State in Iraq and Syria, or ISIS, which has established footholds across northern and eastern Syria with the intention to lay the foundations of an Islamic state.