Opposition fighters have battled rival al Qaida-linked rebels in northern Syria as deep fissures within the uprising erupted into some of the most serious violence between groups opposed to President Bashar Assad since the conflict began.
The clashes, which have spread to parts of four provinces, pit an array of moderate and ultra-conservative Islamist brigades against the Islamic State of Iraq and the Levant, an extremist group that has become both feared and resented for trying to impose its hardline interpretation of Islam.
The fighting did not appear to be a turn in unison by Syrian rebel groups against the Islamic State of Iraq and the Levant, activists and analysts said, but rather an outburst of violence against the al Qaida-linked group in certain communities where tensions with other opposition factions were already simmering.
In a reflection of the fragmented and localised nature of much of the fighting in Syria's civil war, the Islamic State of Iraq and the Levant continued to co-operate with rebel factions against government forces in other parts of the country.
But in some corners of opposition-held northern Syria, the backlash against the Islamic State of Iraq and the Levant has been brewing for months.
The group, which analysts say boasts more than 5,000 fighters, many of whom are foreigners, elbowed its way into rebel-held areas in the spring, co-opting some weaker armed opposition groups and crushing others as it consolidated its grip on new turf.
That infighting has left scores dead on both sides and has undermined the broader rebel movement's efforts to oust Assad. It also has strengthened the government's position ahead of an international peace conference for Syria expected in just over two weeks.
For the West, meanwhile, the rise of the Islamic State of Iraq and the Levant, as well as another al Qaida-linked group, the Nusra Front, has been a source of concern, and a major reason that support in Washington and other Western capitals has dwindled in recent months.
Some in northern Syria originally welcomed the Islamic State of Iraq and the Levant for imposing a degree of order on the villages and towns that fell under its control. But the group alienated many by employing tactics deemed brutal even by the standards of Syria's bloody conflict.
Its fighters have beheaded captured government fighters, and kidnapped anti-Assad activists, journalists and civilians seen as critical of its rule.
The latest and most serious bout of infighting began on Friday after people in the northern province of Aleppo accused members of the Islamic State of Iraq and the Levant of killing doctor Hussein Suleiman.
The newly-created Islamic Front, an umbrella group of powerful, mostly ultra-conservative Islamic fighters, issued a statement ordering the Islamic State of Iraq and the Levant to hand over the doctor's killers so they could stand trial. The extremist group did not, sparking clashes between the factions in Aleppo province.
Fighting quickly spread to rebel-held areas of the north-eastern province of Idlib and the central province of Hama.
Yesterday the violence expanded again, with clashes in the town of Tabaqa in Raqqa province. But much of the heaviest fighting took place in pockets of Aleppo province.
In the town of Manbij, rebels seized a compound garrisoned by the Islamic State of Iraq and the Levant, activists said. The British-based Syrian Observatory for Human Rights monitoring group said fighters from the al Qaida-linked group used car bombs - a tactic usually reserved for attacking government forces - for the first time to defend its territory.
In the town of Tal Rafaat north of Aleppo city, insurgents from the Islamic State of Iraq and the Levant ambushed a rebel convoy, killing at least 14 fighters from the Liwa al-Tawhid brigade, which is a member of the Islamic Front, the Observatory said.
Heavy fighting was also reported in the town of Atareb, several neighbourhoods of Aleppo city itself, as well as in areas of Hama and Idlib provinces.
In total, at least 59 fighters, nine of them from the Islamic State of Iraq and the Levant, were killed Sunday, according to the Observatory.
The Nusra Front, which despite its al Qaida-links has more of a Syrian bent and is seen as more moderate than the Islamic State of Iraq and the Levant, has been trying to mediate an end to the clashes, said Observatory director Rami Abdurrahman .
Some activists hailed the fight against the Islamic State of Iraq and the Levant as a second "revolution," but it seemed unlikely that the battle against the extremist group could unite the constellation of rebel brigades who have failed to forge a unified command over the nearly three-year conflict against Assad.
The Islamic State of Iraq and the Levant is the rebranded version of al Qaida's Iraqi affiliate, which emerged in Iraq's Sunni-dominated Anbar province following the 2003-US led invasion of Iraq.
The Western-backed Syrian opposition in exile has welcomed the fighting against the Islamic State, as it sees the group as hijacking its efforts to overthrow Assad.