BEIRUT, Lebanon -- Competing rebel factions in Syria are increasingly attacking each other in a series of killings, kidnappings and beheadings, undermining the already struggling effort to topple President Bashar al-Assad.
The open hostilities could no longer be contained Friday, when a Western-aligned group, the Free Syrian Army, demanded that a Qaeda-linked rebel faction, the Islamic State of Iraq and Al Sham, turn over the suspected killers of a prominent commander who was shot dead on Thursday. Commanders with the Free Syrian Army warned that the broader movement against Mr. Assad was being threatened by the conflict between itself and the Islamic State.
The infighting is a new low for an opposition that was never able to unite its military or civilian operations. Across the expanse of the battlefield in Syria, in places like the northeastern province of Raqqa and the divided city of Aleppo, rebels are attacking each other and their supporters with regularity and ferocity. Competition for recruits and weapons -- and the right to define the character of the future state -- has fueled the interrebel battles.
"The Islamic State wants to eliminate the Free Syrian Army higher command," Ahmed Farzat, a Free Syrian Army lieutenant, said in an interview on Skype from the central city of Homs, where rebels are struggling against a fierce government assault. "In other words, to marginalize it and replace it."
Kamal Hamami, the Free Syrian Army commander killed on Thursday in the coastal province of Latakia, had just met with others in the group about getting weapons. Mr. Hamami worked as a butcher before the uprising and was one of the first to join it, said Ammar, an antigovernment activist from Latakia who would give only his first name.
But the Islamic State was apparently angry that he was planning an operation without consulting it, said Anas, another activist who witnessed the attack and described it in a video posted online. Mr. Hamami's men, on their way to delivering a Ramadan meal to friends, were blocked by Islamic State fighters angry that a checkpoint had been set up without their permission.
The commanders of the two groups quarreled, Anas said, and the Islamic State commander, Abu Ayman al-Iraqi, shot Mr. Hamami dead.
But the two rebel groups are not just fighting over weapons and tactics.
Last week, members of the Islamic State were accused of beheading two Free Syrian Army fighters and leaving their severed heads beside a garbage can in a square in Dana, a rebel-held town in Idlib Province near the Turkish border. The attack came after clashes broke out at a demonstration against the Islamic State, leaving 13 people dead.
Recently, a fighter from the area, Abu al-Haytham, claimed that the rebel dispute began when a foreign fighter with the Islamic State raped a local boy -- "the last straw," he said -- and Free Syrian Army commanders complained.
"We staged demonstrations to get freedom, not to have an emir ruling us," Mr. Haytham said, referring to the title used by Islamist commanders.
The collection of groups fighting the government has always been an uneasy alliance, and some rebels have long said they expected to battle the more radical groups -- after defeating Mr. Assad -- over their desire to monopolize power and impose religious rule. As the fighting has accelerated, the most radical groups have received the most resources from abroad, allowing them to emerge as the most successful fighting forces.
For a time, that success on the battlefield won the support of many oppositions fighters and activists, who are eager to have a powerful ally. But the prospect of victory has receded as government forces have reasserted themselves with the help of Russia, Iran and the Lebanese militant group Hezbollah. And now some rebels and activists find themselves threatened by fighters they once saw as allies.
"The sea is in front of us, and the enemy is behind us," said Sheik Jassem al-Awad, a tribal leader in Raqqa, adding that he felt squeezed between the government and the radical Islamists. "The Free Syrian Army cannot open two fronts at once."
Sheik Jassem spoke from Turkey, where he fled shortly after being held in a cellar for 25 days by the Islamic State. The group arrested him and eight others from an opposition media center in Raqqa and confiscated $50,000 worth of equipment, he said. One of the others, Jamil Sello, said he had several broken ribs from beatings and had been accused of "trying to establish a secular state, collaborating with the U.S. intelligence and Qatar."
A deputy president of a Syrian tribal union, Sheik Jassem said the Islamists had looted Raqqa of cash and even machinery from its Euphrates River dams. He said that after the merger in April of the Nusra Front, the first radical group to rise within the rebel movement, with Al Qaeda in Iraq, the united group's power had grown "like a larva transformed into a butterfly."
"What can I say?" he added. "The worst thing is that now the regime will gloat."
When the Syrian protest movement became an uprising two years ago, the Free Syrian Army was less an organization than a brand name for a loose collection of units formed by civilians and army defectors. The exile opposition tried to unify them, most recently under the secular-minded Gen. Selim Idris.
But foreign fighters with a steady flow of weapons and cash from radical Islamist donors attracted many Syrians to Nusra, which the United States labeled a terrorist organization for its connection to the Qaeda branch in Iraq.
The group's growing prominence, and the willingness of some Free Syrian Army units to work with it, increased the West's reluctance to provide military aid and gave Mr. Assad an opportunity to paint the entire opposition as driven by foreign-backed extremists.
When the head of Al Qaeda in Iraq declared in April that it was merging with Nusra, some Nusra fighters rejected the merger, and the relationship between the groups is unclear. What is clear is that the proliferation of radical groups of both foreigners and Syrians, which have imposed a harsh interpretation of sharia law and carrying out executions, has angered residents.
Activists this week circulated stories of at least four colleagues arrested by the Islamic State: Zaid Mohammed, who challenged Islamist tactics in Aleppo and was accused of being an apostate; Mohammed Noor al-Matar, who was arrested while protesting with a woman in Raqqa; Abdullah al-Khalil, the head of the civilian council in Raqqa who was trying to establish a police force; and Mustafa al-Ahmadi, who disappeared after being beaten in Aleppo Province.
They also accused the Islamic State of confiscating aid from international aid groups in Tal Abyad in Raqqa Province, in the form of generators to provide clean drinking water and 11,000 food baskets. The group also posted warnings that anyone who violated the Ramadan fast would be detained for the duration of the holy month.
People have protested the Islamic State across the north. "Go back to Afghanistan, you have ruined the revolution," read graffiti in Aleppo.
And in Menbej, in Aleppo Province, after the group erased revolutionary drawings from walls, youths chanted, "Out with the Islamic State, they are no different from Bashar al-Assad."
Hania Mourtada contributed reporting from Beirut.
Correction: July 13, 2013, Saturday
This article has been revised to reflect the following correction: An earlier version of this article misstated the role of the Syrian activist Anas in an online video depicting an attack in the coastal province of Latakia. Anas described the attack in the video but did not post it online.world
This article originally appeared in The New York Times.