On Syria's front lines, al-Qaida fighters and more mainstream Syrian rebels have turned against each other in a power struggle that has undermined the effort to topple Syrian President Bashar Assad.

After violent clashes and the assassination of two rival commanders, one of whom was beheaded, more moderate factions are publicly accusing the extremists of trying to seize control of the rebellion.

The rivalries — along with the efforts by extremist foreign fighters to impose their strict interpretation of Islam in areas they control — are chipping away at the movement's popularity in Syria at a time when the regime is making significant advances on the ground.

"The rebels' focus has shifted from toppling the regime to governing and power struggles," said a 29-year-old woman from the contested city of Homs. "I feel that the lack of true leadership is and has always been their biggest problem." She spoke on condition of anonymity, fearing retaliation from the fighters and the regime.

The infighting, which exploded into the open in the country's rebel-held north in recent days, is contributing to a sense across many parts of Syria that the revolution has faltered. It threatens to fracture an opposition movement that has been plagued by divisions from the start.

The moderates once valued the expertise and resources that their uneasy allies brought to the battlefield, but now question whether such military assets are worth the trouble — not to mention the added difficulty in persuading the West to arm them.

"We don't want foreign fighters. We have enough men and we want them out of Syria," said Brig. Gen. Salim Idris, head of the Supreme Military Council of the Free Syrian Army, an umbrella group for dozens of brigades.

In strikingly blunt comments in an interview with Al-Arabiya on Monday, Idris, a secular-minded army defector who has the backing of foreign powers, accused members of the Islamic State of Iraq and the Levant of being regime agents and "criminals."

That group, formed in April and made up of al-Qaida's branches in Iraq and Syria, has taken on an increasingly dominant role in the Syrian civil war. Many of its fighters are north Africans, Iraqis, Afghans and Europeans who have flocked to Syria to join the overwhelmingly Sunni uprising against Assad.

Gunmen from the Islamic State of Iraq and the Levant were behind the killings of the two rebel commanders, the highest-profile casualties of the growing tensions between jihadi fighters and Western-supported rebels.

Kamal Hamami, known as Abu Basir, served in the Supreme Military Council. Activists say he was shot late Thursday in a clash that erupted after militants tried to remove a checkpoint he set up on the Jabal al-Turkoman mountain in the coastal province of Latakia. Two of his men were seriously wounded in the shooting.

Also last week, members of the extremist group killed Fadi al-Qish, the local commander of a group affiliated with the mainstream Free Syrian Army, or FSA. The fatal attack took place in the village of Dana in the northern province of Idlib near the Turkish border. Activists say the militants decapitated al-Qish and another fighter and left their severed heads on the ground as a lesson to other rebels who challenge their rule in the area.

The executions have enraged FSA commanders, who are demanding that the killers be handed over to stand trial.

Activists also say extremists have recently been sweeping into villages previously controlled by the FSA, taking over crucial resources such as bakeries, oil wells and water pumps to secure people's loyalties. In several cases, the militants were said to seize weapons from army bases and keep them from other rebels.

But what alienates the general population is the brutality. The extremists have carried out summary executions, public floggings and mass arrests, fueling the backlash against them.

In one prominent case in Aleppo last month, al-Qaida-linked militants executed a 15-year-old boy, Mohammad Qattaa, accusing him of being an "infidel" for mentioning Islam's Prophet Muhammad in vain. Gunmen shot the boy dead in front of his parents near a stand where he sold coffee in a killing that sparked rare local protests against them.

In many parts of Aleppo and Idlib and Homs, where a suffocating stalemate has been in place since last year, residents say their support and patience for the rebels is fraying.

In Aleppo last week, residents staged a protest at a checkpoint against a blockade imposed by the militants on government-held districts, because the blockade created food shortages at the onset of Ramadan, the Muslim holy month. The protest led to a physical quarrel between supporters and opponents of the siege and ended with gunshots fired in the air to disperse protesters.

Syria's uprising started in March 2011 as an Arab Spring-inspired revolt against the decades-long Assad family rule. It eventually transformed into an insurgency and civil war in response to a brutal government crackdown against the protests. More than 93,000 have been killed and millions uprooted from their homes.

The rebels are a disparate mix of ordinary citizens who took up weapons, army defectors, moderates and hard-liners, and increasingly, jihadists who have trekked to Syria from all over the world. A shortage of weapons and the inability of external players to interfere in the conflict to tip the balance in favor of one side or another has worked against the rebels.

Some FSA commanders are trying to tamp down the dispute with the al-Qaida militants, mindful of the damage the infighting has done to their cause.

"Their actions are despicable, but we will not be drawn into a fight with them," said one commander, who declined to be named so as not to aggravate the situation.

FSA spokesman Loay al-Mikdad was less delicate.

"I think they should come out in public and tell the Syrian people why they are in Syria. Is it to fight Bashar Assad or to impose a specific agenda on the Syrian people?" he asked.

"We never see them on the battlefield anymore," he said of the al-Qaida militants. "We only see them in liberated areas either next to oil wells or trying to impose specific agendas on territories."

The dispute is not restricted to Islamic militants versus moderates. In the north, there has also been deadly infighting between Kurdish and Arab groups over control of captured territory along the border with Turkey.

"This infighting is very dangerous and is undermining our revolution," said Mohammed Kanaan, an activist based in the northern province of Idlib. "People are fed up and tired. ... They are starting to hate both sides," he said via Skype.

Rami Abdul-Rahman, director of the U.K.-based Syrian Observatory for Human Rights, said the al-Qaida militants are working to entrench themselves and secure a place in a post-Assad Syria.

"They are trying to control everything, they have a lot of money," most of it from private donations, he said.

Still, al-Mikdad ruled out a scenario similar to the Iraqi one, when U.S.-allied groups of Sunni fighters battled al-Qaida.

"Until now, the FSA does not consider itself in confrontation with these groups. Our weapons are directly only against Bashar Assad's troops," he said in a TV interview.

"But if a fight is imposed on us, we will defend ourselves," he said.

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AP correspondent Yasmine Sakr contributed to this report.