The most feared and effective rebel group battling President Bashar al-Assad, the Islamist Nusra Front, is being eclipsed by a more radical jihadi force whose aims go far beyond overthrowing the Syrian leader.
Al Qaeda's Iraq-based wing, which nurtured Nusra in the early stages of the rebellion against Assad, has moved in and sidelined the organization, Nusra sources and other rebels say.
Al Qaeda in Iraq includes thousands of foreign fighters whose ultimate goal is not toppling Assad but the anti-Western jihad of al Qaeda leader Ayman al-Zawahri - a shift which could extend Syria's conflict well beyond any political accord between Assad and his foes.
The fighting has already cost 90,000 lives. The break-up of an important part of Syria's opposition, already splintered into hundreds of armed groups, worsens the dilemma faced by the West as it debates whether intervention to support the rebels will result in arms being placed in the hands of hostile Islamist militants.
And if the West were to intervene, it may now be under pressure to attack al Qaeda opposition forces rather than Assad. "Nusra is now two Nusras. One that is pursuing al Qaeda's agenda of a greater Islamic nation, and another that is Syrian with a national agenda to help us fight Assad," said a senior rebel commander in Syria who has close ties to the Nusra Front.
"It is disintegrating from within." Others said that Nusra's Syrian contingent has already effectively collapsed, with its leader Abu Mohammad al-Golani keeping a low profile and his fighters drifting off to join other rebel groups.
Nusra fighters have claimed responsibility for the deadliest bombings of the two-year-old Syrian conflict and their brigades have led some of the most successful rebel offensives against Assad's forces.
The group was formally designated a terrorist organization by the United States six months ago, a step which Washington said was vindicated by a declaration in April that it was merging with al Qaeda's Islamic State of Iraq. Washington now says Nusra is little more than an al Qaeda front.
A U.S. official said on Friday: "We continue to be concerned about the influence of extremist groups, including al Qaeda in Iraq. This is why we have been coordinating and discussing with partners the need to continue to strengthen the moderate opposition and channel any assistance through the moderate opposition, including the Supreme Military Council."
The U.S. moves to isolate Nusra have drawn criticism from Syrian rebels and opposition leaders, reflecting the fact that Nusra was able to win grudging support beyond its core Islamist base because of its fighters' discipline and battlefield successes.
Many Syrians turned a blind eye to the growing presence of foreign and Arab jihadi fighters in its ranks because Nusra fighters cooperated with other rebel brigades, worked to curb looting and provided help for displaced Syrians.
By contrast the head of the Islamic State of Iraq, Abu Bakr al-Baghdadi, who has moved into northern Syria to take tighter control over al Qaeda operations in the country, has few admirers among Syrian fighters.
They see him as a brutal figure with little time for the intricacies of Syria's struggle, focused less on toppling Assad and more on imposing a radical Islamist rule including religious courts and public executions.
Many accuse him privately of hijacking their revolution. "We reject his presence here on the ground. He should take his fighters and go back to Iraq," said a Nusra source who is close to Nusra leader Abu Mohammad al-Golani.
"We are not happy with the way he operates nor with his methods." Baghdadi's announcement in early April that his Islamic State of Iraq was formally merging with Nusra to form the Islamic State of Iraq and the Levant clearly took the Syrian Nusra rebels by surprise.
But the move did not help. Soon after, in a direct challenge to Golani, Baghdadi traveled from Iraq to a town in Syria's Aleppo province, where he was joined by Arab and foreign jihadis who had formerly fought for Golani's Nusra.
Rebels say the rift continued to widen and the foreign and Arab wing is now operating formally under the banner of the Islamic State of Iraq and the Levant, while many Syrian Nusra fighters have dispersed to join other Islamist brigades. "The situation has changed a lot. Baghdadi's men are working but Nusra is not working formally anymore," said another Nusra source.
"Those with Baghdadi are the fiercest fighters of all. The brothers are trying to avoid them as much as possible."
The source, and other Syrian Nusra fighters who spoke to Reuters, said they feared Baghdadi's supporters would alienate Syrians in the same way that their hardline agenda turned Iraqis against them, paving the way for U.S.-backed Sahwa militias to turn the tide against al Qaeda in western Iraq in 2007.
Nusra sources said they were waiting for Zawahri to settle the issue, hoping he would call on Baghdadi to return to Iraq. "We have two choices now. Either Zawahri announces the separation of Syria's Nusra from Iraq's Islamic State, or he orders Baghdadi to stay (in Syria) and if this happen then its a disaster," said one Nusra source.
"Baghdadi has harmed the Nusra Front. He caused great damage and broke up the front." But the Syrian rebel commander, who is from a Western-backed rebel group, said that Baghdadi already had Zawahri's blessing when he moved in. "They set a trap for Golani," he said.
"They wanted a foot inside (Syria) and helping Golani at the start with men and arms provided that, until they became stronger so they took over." In a telling video published this week, masked fighters from the Islamic State of Iraq and Levant executed three men they said were officers from Assad's Alawite minority sect in the eastern town of Raqqa.
After the shootings there were only muted chants of support for the fighters and activists said that small protests broke out at night condemning the execution and calling on the fighters to fight Assad instead of executing people.
Several Nusra fighters said they feared that if Baghdadi's influence continued to grow, his ultra-radical agenda would see the Iraqi Sahwa phenomenon played out again in Syria. "We as Syrians do not want a repeat of that. The Baghdadi men have declared the Nusra fighters who left him... as infidels. We still reject his state, if Zawahri does not put an end to this then the situation will get worse," one said.
The senior rebel commander said he even expected the growing clout of Baghdadi's fighters would finally end the West's reluctance to intervene militarily in Syria - not against Assad, but his hardline enemies. "We expect soon drone attacks, like Yemen, to begin against al Qaeda members," he said.
Meanwhile Golani, who was formally declared a terrorist by Washington on Thursday, is now in hiding. "He has gone to ground until the problem is solved," said a source close to Golani. Even though few people even know what Golani looks like, and fewer still have met him, he has gained popularity among Syrians and songs have been written to celebrate his exploits.
His real name is not known even to some of his fighters and many people long suspected that he was a fictitious fighter created to give a Syrian 'front' to the Iraqi al Qaeda. Sources say he is Syrian and in his 40s, roughly the same age as Baghdadi.
He is currently in rural Damascus province, they say, accompanied by some of his remaining fighters. Baghdadi, an Iraqi, helped fund Nusra fighters, who also had financial support from private donors in Arab Gulf countries. The Iraqi wing is financed from al Qaeda's global support network.
One Nusra fighter said he believed Baghdadi held a personal grudge against Golani because of his standing in Syria. Golani, a radical Sunni Muslim, won popularity in Syria even among some Christians, according to the Nusra fighter. "Baghdadi did not like this," the fighter said.
"Baghdadi and the (al Qaeda) leadership consider the Muslim Brotherhood, the Free Syrian Army and other factions including Christians as infidels and when they saw Golani was on good terms with them they were not happy."
"That is why he announced the Islamic State of Iraq and Levant without any consultation with Golani, and he is in charge to operate in his old failed way."
By Mariam Karouny