A car bomb blamed on a radical group linked to al Qaeda has killed at least 26 people in a rebel-held city of northern Syria.
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 denied the death.
An array of Syrian rebel 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 three-year-old uprising, with hundreds of rebels killed.
The Syrian Observatory for Human Rights said most of those killed in the car bomb in Jarablus were from rival rebel groups, though it counted at least three 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 province, though they have recaptured much of the northern province of Raqqa and other parts of Aleppo province.
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 rebel checkpoints or even flee towns that were likely car bomb targets.
Rebels hold large swaths of territory in northern and eastern Syria but their infighting has put those gains at risk - Assad clawed back some territory east of Aleppo, Syria's second city, earlier this week.
BELGIAN FIGHTER KILLED IN SYRIA
Elsewhere in the rebel-held north, local activists said the Belgian commander of al Qaeda-linked forces in Syria was killed in fierce clashes between rival rebel groups for the northern town of Saraqeb.
ISIL supporters, however, denied reports that the local Saraqeb "emir", known as Abu Baraa al-Jazairi, had been killed.
Activists said rival rebels 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, the coastal stronghold of President Bashar al-Assad.
"The Islamic Front is playing 'all in' at Saraqeb ... it is a strategic area they want, whatever it costs. They've been trying to control it for a week," said a rebel source with ties to hardline Islamist groups, who declined to be named.
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 rebels 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 rebel 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 rebel 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.