Syria rebel group demands Islamists turn over suspected killer BEIRUT - The principal U.S.-backed Syrian rebel group is demanding that an Islamist insurgent faction turn over the suspected killer of one of its commanders in a slaying that has highlighted tension among allies fighting to overthrow the Syrian government. "We demand that the perpetrators of this heinous crime be handed over to be tried by an independent judicial commission," Louay Meqdad, a spokesman for the Free Syrian Army, said Saturday in a telephone interview from Istanbul, Turkey. If they don't, then we will take measures. The spokesman did not elaborate on the "measures," but the slaying last week of Kamal Hamami, apparently by an Islamist rebel group, has spurred outrage among the Free Syrian Army leaders. The group has laid blame for the slaying on a powerful rebel faction, the Islamic State of Iraq and Syria, which is linked to Al Qaeda. Hamami was shot to death Thursday in a rebel-held stretch of northwest Syria in what appeared to be a turf dispute between the two insurgent groups, according to various accounts. The incident has prompted fear of a "civil war within the civil war" should Free Syrian Army and Al Qaeda-affiliated rebel groups enter open conflict. But Meqdad, the Free Syrian Army spokesman, did not directly threaten retaliation, demanding only that the suspect be handed over. "We at the FSA are not after attacking anyone but the regime," Meqdad said, referring to the government of President Bashar Assad. But if anyone believes that they can attack the cadres of the FSA without a response then they are wrong. How far-reaching any retaliation could be remains a question. The rebel group has a loose command structure and is more of a franchise operation than a top-down military command. It is not clear how many rebel brigades identifying themselves as affiliated with the Free Syrian Army actually take direction from the exiled leadership based in Turkey. Many commanders in the field say they act in autonomous fashion and take no orders from officers based abroad. Moreover, even many affiliated insurgents have a heavily Islamist identity and have adopted Islamist titles for their brigades and battalions. The group's units and unaffiliated Islamist factions have often collaborated in the more than two-year struggle to topple Assad. Commanders have complimented the fighting prowess of rebel groups like Al Nusra Front, which the Obama administration has labeled an Al Qaeda-linked terrorist organization. The ranks of Al Qaeda-linked groups like Al Nusra include many foreign militant volunteers from the Arab world, Europe and elsewhere. But tension has surfaced about the differing view of a future Syria envisioned by the Free Syrian Army's leadership and by conservative Islamist factions like Al Nusra and the Islamic State of Iraq and Syria. The more Islamist groups generally seek to install a theocratic state in Syria, while the Free Syrian Army leadership says publicly that it wants a secular democracy once Assad's government is toppled, an outcome that is in no way assured. Government forces have made major territorial gains against the opposition. While the rebels are fragmented, the government has the advantage of a professional army unified behind Assad's leadership. There have been several reports recently of battles between Free Syrian Army units and Islamist factions. On Saturday, a pro-opposition monitoring group, the Syrian Observatory for Human Rights, reported clashes between rebel groups in the northern province of Idlib. But there were no details about the reported battles. One factor in the tension between the rebels may be demands placed on them by their foreign backers. All depend heavily on foreign aid, either from governments like Saudi Arabia and Qatar or from wealthy sympathizers abroad. The United States and its European allies have said they are contemplating providing arms to "moderate" Syrian rebels fighting under the Free Syrian Army banner. But many U.S. officials have voiced concern that any donated weapons may end up in the hands of ardently anti-Western Al Qaeda-linked fighters. Many worry that the distinction between rebel units in the field is often blurry. Cognizant of such doubts, the Free Syrian Army leadership has tried to make the point repeatedly that the group's operations are separate from those of hard-line Islamist rebel factions linked to Al Qaeda. Bulos is a special correspondent.