A Belgian commander of al Qaeda-linked forces in Syria was killed in fierce clashes between rival rebel groups for the northern town of Saraqeb on Wednesday, activists said.
Supporters of the Islamic State of Iraq and the Levant however denied reports that the local Saraqeb "emir", known as Abu Baraa al-Jazairi, had been killed.
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-Qaedawith a core of foreign fighters.
According to the Syrian Observatory for Human Rights, 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.
More than a week of internecine clashes has killed hundreds of rebels, in one of the bloodiest episodes of infighting since the nearly three-year-old uprising began.
The alleged Jazairi killing comes amid heavy fighting for ISIL-held Saraqeb, which straddles three highways that lead to the capital Damascus, Syria's second city 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-Qaedagroups 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-Qaedain other war zones, have regained lost ground in recent days.
Assad's forces have made some advances in the rebel-held swathes of northern Syria east of Aleppo city due to the rebel infighting, pro-government media said.
In neighbouring Lebanon, the military said it had captured a leader of another al Qaeda-linked militant group.
Jamal Daftardar, a Lebanese citizen, was a leading figure in the Abdullah Azzam Brigades, which claimed responsibility for a bombing of the Iranian embassy in November which killed at least 23 people, it said.