BEIRUT, Lebanon -- The deadly clashes that raged between rival rebel factions in Syria over the weekend accentuated the divisions hampering opponents of President Bashar al-Assad as they try to halt his forces' recent gains on the battlefield and persuade the West to supply the insurgency with weapons.
The fighting flared between members of a mainstream rebel group and a radical faction affiliated with Al Qaeda, according to local residents and an antigovernment watchdog group. The presence of the radicals and the failure to bring rebel forces under a unified military leadership have made the United States and its allies reluctant to arm the opposition.
As foreign fighters continued to spill into Syria across the country's porous borders -- committing atrocities against both supporters and opponents of the government and clashing with more moderate rebel groups -- the prospects for unity among the rebels have seemed to grow more remote.
Islamist fighters said recently that they had driven a rival rebel brigade out of Raqqa, a rebel-held provincial capital in northeastern Syria, because they had found some of its fighters drinking wine and consorting with women, and because they considered brigade reluctant to fight.
And in the most recent confrontation, in Dana, a rebel-held town in Idlib Province near the Turkish border, members of an extremist Islamist group were accused of beheading two rival fighters and leaving their severed heads beside a garbage can in a town square.
That grim discovery on Sunday followed a protest and clashes in the town, highlighting the antagonism that some Syrian fighters and civilians are beginning to feel toward some of the radical factions.
Simmering tensions within the insurgency have intensified lately, especially between rebels affiliated with the Free Syrian Army, the loose-knit umbrella group backed by the West, and members of the Islamic State of Iraq and Al Sham, the new Syria-based affiliate of Al Qaeda, which includes the well-armed fighters of the Nusra Front.
The disputes that have erupted in provinces like Raqqa, Idlib and Aleppo are often centered as much on individual egos and economic resources as on ideology. Still, the frictions inhibit the insurgency from functioning as a single fighting force, and as government forces have appeared to retake control of some rebel-held territory, they are threatening to deepen into a new conflict that would further weaken the rebels on the battlefield.
According to residents, activists and fighters, popular resentment rose as radical factions, including foreign fighters, began sweeping into villages that had been under the control of the Free Syrian Army and trying to impose their strict conception of Islamic law, sometimes carrying out summary public executions.
The Nusra and Free Syrian Army fighters have accused each other of profiting from the war, demanding bribes at checkpoints and selling oil from captured wells. The Qaeda-affiliated units are said to be buying up land in Aleppo and Idlib Provinces and trying to monopolize supplies of wheat and fuel, Reuters reported.
The complaints run in both directions. In Raqqa, Abu Abdullah, a fighter with the Islamist brigade of Ahrar al-Sham, said his men, along with Nusra fighters, had ejected the Farouq brigade from the city for both military and moral reasons.
He said the Farouq brigade was hoarding arms and had failed to go to the aid of allies in Qusayr, a city that government forces retook last month.
"We want real fighters," he said in an interview, "not lazy people who raise the revolution flag but spend time relaxing and getting money."
In Idlib Province, residents of Dana staged a demonstration on Friday to demand that Qaeda militants leave their town, because the Qaeda fighters had been detaining, beating and in some cases executing teenage boys for trivial offenses, according to a published statement by Syrian activists who identified themselves with the phrase "Al Nusra Front does not represent me."
It was not clear who had instigated the clashes, which claimed a number of lives, according to the Syrian Observatory for Human Rights, an antigovernment watchdog group that tracks violence in the country from a base in Britain.
According to a local Free Syrian Army commander and the Observatory, the Qaeda fighters opened fire at the protesters, prompting return fire from more moderate fighters. The civilian demonstrators were caught in the middle, the Observatory said, and several civilians were killed, some of them children.
But a local group calling itself the Union of Free Youths of Idlib said that the Qaeda fighters had blamed others for shooting first, and that the clashes had been between disreputable members of battalions whose more serious fighters were busy on the front lines.
Those who fought in Dana "have nothing to do with jihad, except they like showing off," the group said on its Facebook page. "We wouldn't say one side is Islamic and the other is secular. Both sides are only fighting for personal gains and glory, while martyr after martyr is falling on the front lines."
The local Free Syrian Army commander said that after the clashes in Dana, members of the Qaeda group had begun seizing people in the town, including Fadi al-Qesh, the leader of the Hamza Assad Allah brigade, a Free Syrian Army affiliate. The bodies and severed heads of Mr. Qesh and his brother were found later.
In a video circulated by the Observatory, a man with his face obscured, identified as another Free Syrian Army commander, is heard to say ruefully that when the radical fighters first arrived in the town, "we warmly welcomed them -- we opened our homes and even participated in battles with them." Now that camaraderie has apparently frayed.
Hwaida Saad contributed reporting from Beirut, and an employee of The New York Times from Raqqa, Syria.
This article originally appeared in The New York Times.