Molhem Barakat / Reuters Free Syrian Army fighters call out to forces loyal to Syria's President Bashar Assad, urging them to defect, as another fighter stands guard in the old city of Aleppo on Sept. 1, 2013

Ongoing clashes between rival groups within the armed opposition intensified in Syria’s Aleppo province this past week following protests against the heavy-handed tactics of the al-Qaeda-affiliated Islamic State of Iraq and Syria (ISIS). Infighting among rebels could spell trouble for an opposition movement seemingly on the wane, but it could also present an opportunity. If the moderate-leaning rebel groups can sever their symbiotic relationship with their al-Qaeda affiliates for good, they stand to get significantly more support from Western backers wary of inadvertently assisting old enemies. But it won’t be easy — even as the rivals battle for turf in Aleppo province, they have united to inflict a resounding defeat on government forces elsewhere in the country.

For the past several months rebel groups aligned with ISIS in Aleppo province have spent nearly as much energy battling factions serving under the umbrella of the Western-leaning Free Syrian Army (FSA) as they have fighting the regime of Syrian President Bashar Assad. According to al-Quds al-Arabi, an Arabic-language newspaper published out of London, the media office of ISIS issued a statement on Sept. 12 saying it had launched a military campaign against FSA battalions in Aleppo province in response to a previous attack on the ISIS headquarters there. The most recent clashes, which took place in Bab, a district 25 km from the provincial capital, were sparked by an anti-ISIS rally. Enraged, members of the fundamentalist group shot into the crowd, injuring eight, says Abu Mohammad, an engineer who was at the scene.

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Abu Mohammad, who spoke to TIME via Skype, estimates that over the past several months similar clashes have resulted in dozens of dead — and none of the victims were members of the Syrian government that the rebels are ostensibly seeking to overthrow. “People are fed up with their behavior. Anyone who disagrees with them is an infidel. Any moderate person is an infidel. Simply if you are not with them you are an infidel,” he says, asking to be identified only by his nickname, for his protection. The infidel accusation, according to ISIS’s draconian interpretation of Islamic law, can result in execution.

Abu Sohaib, an ISIS leader in the nearby town of Azaz, was not involved in the Bab conflict, but he defends the groups’ new campaign, telling TIME via Skype that ISIS is only fighting collaborators and war profiteers, not critics. He says most of the clashes started because members of the FSA were stealing money, humanitarian supplies and food aid destined for civilians. “People are fed up from the so-called FSA, because of their corruption and behavior. We launched the campaign to purify the revolution, and so far we have succeeded in eliminating most of those who profit from the revolution.”

Abdul Rahman Mattar, an Aleppo-based human-rights activist and writer, agrees that ISIS and its ideological sibling Jabhat al-Nusra, which is listed as a terrorist organization by the U.S. State Department, were initially welcomed in the area for their superior fighting skills and low tolerance for the petty corruption of FSA soldiers. But, he adds, speaking to TIME via Skype, their harsh interpretation of Islamic law is starting to grate on local Syrians who don’t share that ideology. “People don’t want to replace one dictatorship with another,” says Mattar. “One of the main problems of ISIS in Syria is that many of its fighters are outsiders, and they don’t understand Syrian society.” Still, he adds, “I cannot understand how they forgot about fighting the regime and focused instead on fighting the rebels.”

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To Paul Salem, director of the Carnegie Middle East Center, an international-affairs think tank based in Beirut, the widening schism between ISIS and other, more moderate elements of the opposition is a good thing, even if it temporarily distracts from the battle against Assad. “The rise of [Jabhat al-Nusra and ISIS] groups is a far bigger threat to the region than Assad staying on for a few more years in Damascus,” he says, noting that the Assad regime — despite its oft-vocalized fears of a jihadist takeover — has notably refrained from attacking ISIS targets, the better to undermine the rest of the rebel groups. The Supreme Military Council (SMC), the Western-backed group that is attempting to organize the FSA, understands the importance of getting rid of ISIS, says Salem. “They know [the rebels] can’t defeat the regime alone, and they know that the U.S. won’t let them win if that means letting Jabhat al-Nusra win. So they have realized that strategically speaking, Nusra is a no go. So they have to divorce.”

But as any recently divorced couple knows, untangling assets can be messy. ISIS groups and FSA brigades may be at one another’s throats in Aleppo, but in Maaloula, a strategic town near Damascus, Jabhat al-Nusra worked seamlessly with FSA member brigades to achieve a decisive victory against the regime last weekend. The joint effort raises questions of just how possible it really is to cleave the pro-Western rebels away from the rest

Maaloula is the rule and Bab is the exception, according to Swedish analyst and researcher Aron Lund, who has just completed a study of Syria’s nonstate actors. It is very rare that one group pulls off an operation all by itself. Instead individual commanders from across the ideological spectrum join forces to plan the attack, each contributing its specific expertise and weapons cache. “It’s unrealistic to expect that you can tell rebels to stay away from other rebels as long as civil war is going on.” Al-Nusra, he points out, has a particularly effective battlefield weapon that few of the more moderate groups can claim: suicide bombers. The conquest of Maaloula started with a suicide attack by an al-Nusra fighter on the government checkpoint. The fight went downhill from there. “The suicide attack is a powerful weapon,” says Lund. “Many battles start with an operation to take the military base. To do that you have to break the perimeter, and you can’t do that without a suicide bomber. So that is what Nusra brings to the table.”

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The FSA’s battlefield deficiencies, so to speak, can be overcome if it gets better funding and more supplies, says Salem. But it won’t, and shouldn’t, happen overnight. What’s at issue now is not so much arming the SMC to make a decisive push against the Assad regime as turning the group into a credible military and governance entity. The broader objective should be to stabilize the rebel areas, to open schools, to have medical clinics and to bring in governance, he says. Only then should rebels start talking about defeating the regime. “If you can have a credible SMC in a few years, one that can bring governance, that has a military capacity, and that can control the jihadis, then the West might start saying, O.K., we can afford for Assad to fall.” That will have an impact on the Syrians too, he adds. “Right now they are saying, ‘If the opposition is going to be Jabhat al-Nusra, frankly we prefer Assad.’ But if the SMC can get its act together, and it can assure Syrians that it is strong and capable and that they won’t be harassed by these jihadis, then they will support it.’” For the moment, Syrians only see a choice between ISIS and Assad. But if the SMC can step up, the town of Bab might unite to fight the regime, instead of one another.

— With reporting by Rami Aysha / Beirut