A car bomb blamed on a radical group linked to al-Qaeda killed at least 26 people in northern Syria on Wednesday.
Hours earlier, a Belgian commander from the same al Qaeda-linked group, the Islamic State of Iraq and the Levant (ISIL), was reported killed in northern Syria.
ISIL supporters, however, denied reports that the local Saraqeb "emir", known as Abu Baraa al-Jazairi, had been killed.
Activists said rival militants ambushed a convoy of ISIL fighters and killed Jazairi, who is believed to be a Belgian citizen of Algerian origin.
Belgium's foreign ministry said it was aware of the reports of Jazairi's death but could not confirm them.
The alleged Jazairi killing comes amid heavy fighting for ISIL-held Saraqeb, which straddles three highways that lead to the capital Damascus, Aleppo and Latakia.
An array of Syrian militant groups, including the large alliance known as the Islamic Front, have been trying to push out ISIL, a small but powerful affiliate of al Qaeda with a core of foreign fighters.
The fighting has sparked the bloodiest internecine clashes in the history of Syria's nearly 3-year-oldcrisis , with hundreds of militants killed.
The opposition observatory said most of those killed in the car bomb in Jarablus were from rival militant groups, though it counted at least 3 civilians. It said the death toll was likely to rise, as dozens more were severely wounded.
ISIL fighters have been losing territory in the area around Jarablus, in the northern Aleppo governorate , though they have recaptured much of the northern governorate of Raqqa and other parts of Aleppo.
The group has vowed to use tactics such car bombs against its rivals, and some Syrian ISIL members have used social media to warn fellow Syrians to avoid militant checkpoints or even flee towns that were likely car bomb targets.
Western powers have reduced their support for the Syrian opposition due to the advance of radical al Qaeda-linked groups.
Some observers suspect that Gulf Arab countries bankrolling the militants are pressuring them to weaken al Qaeda groups in return for support.
The conflict has created an alliance of several non-ideological groups with the Islamic Front, the largest amalgam of militant forces in Syria, though not all of its units are supporting the campaign to flush ISIL out of its northern bases.
ISIL is the reincarnation of al Qaeda's branch in Iraq. Its numbers are smaller than other militant groups but its battle-hardened militants, many of them foreigners with experience fighting with al Qaeda in other war zones, have regained lost ground in recent days.