Jabhat fighters disillusioned after apparent takeover by hardline jihadists from Iraq.
Jabhat al-Nusra, the much-feared militant jihadist group that has taken control of large rebel-held areas of northern Syria, has split in two following its leadership's public declaration of allegiance to al-Qaeda.
Some of its fighters have withdrawn from the front line against the Assad regime in the northern Syrian city of Aleppo, rebel leaders there have told the Daily Telegraph, and appear to have turned their back on their Syrian leader.
Many Jabhat fighters had been recruited from other, rival militias with the promise of better-funded and better-organised units rather than for ideological reasons.
But they are said to have become disillusioned since their Syrian leader, Abu Mohammed al-Jolani, affirmed his loyalty to al-Qaeda after an apparent takeover at the top of Jabhat by hardline jihadists from Iraq.
"The group has split," Mohammed Najib Bannan, the head of the Aleppo Judicial Committee's military arm, said.
The committee is backed by the major rebel brigades and runs civil and criminal courts in Aleppo alongside the city's sharia court.
"Some of Jabhat al-Nusra supported al-Jolani's statement, and some disagreed," he said.
Jabhat has been under growing pressure since the United States declared it a proscribed terrorist group and United Nations diplomats are pushing for it to be subject to the same sanctions as apply to any other part of al-Qaeda.
Jabhat fighters in the east of the country had started calling themselves the "Islamic State of Iraq and Syria" on videos posted online - the name preferred by the international al-Qaeda leadership. They include a group that publicly executed three regime officers in the town square in Raqqa last week.
Ayman al-Zawahiri, al-Qaeda's leader since the death of Osama bin Laden, appointed Abu Bakr al-Baghdadi, the former head of the Islamic State of Iraq, head of a new merged organisation. Al-Jolani apparently acquiesced.
Baghdadi is regarded as committed to using intense violence for sectarian purposes and to break down society to give "space" for jihad to flourish. He proclaims the need for an international caliphate, ending national borders.
Many Jabhat fighters say they are Syrians first and joined Jabhat just to rid the country of President Bashar al-Assad, and because the brigade was considered less corrupt and more religious.
The split was confirmed by other activists and officials belonging to both civilian units running services in Aleppo and military brigades. The latter also commented on their disappearance from much of Aleppo.
Abdulaziz "Abu Jumaa" al-Salameh, political leader of the biggest brigade in Aleppo, Liwa Tawhid, said that after months in which men had defected to Jabhat al-Nusra, the tide had begun to turn in the opposite direction. He said in recent days an entire unit of 120 men had left Jabhat al-Nusra to rejoin Tawhid.
He said military relations continued with Jabhat al-Nusra, but non-military relations had ceased.
The West fears that the Syrian conflict might come to replicate even more the murderous sectarian carnage of post-invasion Iraq.
In much of northeast Syria up to the Iraqi border, Jabhat and another radical militia, Ahrar al-Sham, dominate the rebel movement and have arrested rival brigade leaders.
Last week they paraded three hooded men they said were Assad soldiers in the square before shooting them in the back of the head. The executioner said he represented the "Islamic State of Iraq and Syria".
Aymenn Jawad Al-Tamimi, a researcher on Arab politics, said recent videos posted online claiming allegiance to the new organisation were largely from the east, while the Jabhat al-Nusra designation survives in those from the west.
"All this evidence suggests that Jabhat's name and symbols are beginning to be replaced by that of ISIS, and this trend has become particularly apparent in the east of the country," he wrote on the Brown Moses blog, which analyses videos posted from the Syrian conflict.
In-fighting among rebel groups has proved a major blockage to further advances since a surge across the north of the country which lasted until three months ago.
One rebel militia leader in Aleppo said hostilities against the regime had been put on hold all last week while a group of brigades carried out an order to expel a militia called Ghoraba al-Sham which had been accused of "going rogue" and indulging in a looting rampage.
By yesterday, 200 of its members had been rounded up and jailed, according to rebel officials, and it had been removed from checkpoints at a main entrance to town which was closed briefly last Thursday during fighting between the two sides.