BEIRUT, Lebanon - Days of clashes between rebel factions in northern Syria have spread to the eastern city of Raqqa, once a bastion of support for the al-Qaeda-linked Islamic State of Iraq and Syria (ISIS) activists said.
Activists and rebel leaders also report that ISIS has withdrawn some of its forces from a number of towns in northern Syria.
The rebel infighting is some of the most significant of Syria's nearly three-year-long crisis, which began with an uprising against President Bashar al-Assad’s regime. ISIS is locked in fighting with the Islamic Front, the country’s largest rebel alliance, and the Western-backed Free Syrian Army (FSA). Reports suggest the violence has left dozens of rebel fighters killed or captured.
The trigger for the latest flare-up appears to be the murder of Hussein al-Suleiman, also known by his nom de guerre "Abu Rayyan," a physician by training and a commander of Ahrar al-Sham, a powerful Islamist militia that is a member of the newly-formed Islamic Front alliance.
They kidnapped him and tortured him and then killed him in a way that wasn't known by the Syrian people before the revolution, 
The Islamic Front says Suleiman was arrested while meeting with members of ISIS to settle a local dispute. A few days later his dead body was returned to the Front, showing marks of torture. A gruesome photograph was quickly circulated on the Internet.
"They kidnapped him and tortured him and then killed him in a way that wasn't known by the Syrian people before the revolution," said the Islamic Front in a statement, demanding that ISIS hand over the men responsible for Suleiman's death.
Bringing Suleiman's killers to justice was the theme of the following Friday's protests, and the Western-backed Syrian National Coalition issued a statement condemning ISIS as a "terrorist organization with close connections to the terrorist Assad regime."
But the apparent murder was merely the spark that ignited long-simmering tensions between ISIS and rival rebel groups. For months, ISIS antagonized other rebel groups as it attempted to enforce its strict vision of Islamic law. ISIS has cracked down on activists, carried out executions, instituted strict dress codes for women and imposed smoking bans.
In November, ISIS gunmen in Aleppo mistakenly executed a man they believed to be connected to the Syrian government. The gunmen heard him muttering prayers that they believed identified him as a Shiite, the minority Muslim sect to which Assad belongs. The fighters decapitated the man and held his head up to a small crowd. After a video of the incident circulated on social media sites, it turned out that the man was in fact a commander of another Islamist rebel brigade.
Unlike previous isolated clashes, this recent wave of violence appears to be a more organized – and growing -- push against ISIS. Following the first day of clashes, a new umbrella group of Islamist fighters, the Army of the Mujahideen, declared its formation in opposition to the al-Qaeda-linked group.
"We, the Army of the Mujahideen, pledge to defend ourselves and our honor, wealth and lands and to fight ISIS which has violated the rule of God, until it announces its dissolution," the new alliance said in a statement released on Facebook.
Three days later, the Islamic Front issued a statement threatening ISIS. "We fight against whoever attacks us and whoever pushes us to battle, whether they are Syrian or foreign," the Front said.
We, the Army of the Mujahideen, pledge to defend ourselves and our honor, wealth and lands,
Rebel groups say they have ousted ISIS members from parts of Aleppo and Idlib provinces in the north, and that clashes are ongoing in Raqqa to the east and Hama to the south.
The Syria Observatory for Human Rights, an anti-government group that monitors the Syrian crisis through a network of contacts, said that clashes between ISIS and Islamist battalions were ongoing in Raqqa city on Monday.
"The scale and breadth of these clashes are unprecedented," said Noah Bonsey, a senior analyst at the International Crisis Group. "We're seeing a more organized, broader campaign against ISIS,” Bonsey added. “For the Islamic Front to put out a statement clearly taking the side of the (Western-backed) rebels against ISIS is notable."
Bonsey said that tensions between ISIS and other rebel groups were exacerbated in part due to ISIS' own rise. "As ISIS seeks to expand its zone of control into other rebel held areas, they do so at the expense of other rebel groups and they're quite willing to confront other rebel groups."
While some rebel groups welcomed the confrontation with ISIS, inter-rebel fighting ultimately saps the wider opposition aim to bring down the Assad government. On the battlefield, resources are being diverted, and at the negotiating table the violence could strengthen Assad's hand.
