* Infighting between hardline Islamist rebels intensifies
* Killing likely to provoke revenge attacks
* Death of al Qaeda fighter a gift to US, rebel says (Adds detail on al-Soury)
By Mariam Karouny
BEIRUT, Feb 23 (Reuters) - A Syrian rebel commander who fought alongside al Qaeda founder Osama bin Laden and was close to its current chief, Ayman al-Zawahiri, was killed by a suicide attack on Sunday, intensifying infighting between rival Islamist fighters.
The Observatory for Human Rights in Syria said Abu Khaled al-Soury, also known as Abu Omair al-Shamy, a commander of the Salafi group Ahrar al-Sham was killed along with six comrades by al Qaeda splinter group the Islamic State of Iraq and the Levant (ISIL). It said al-Soury had fought in Iraq and Afghanistan.
Al-Soury's death will fuel the infighting among jihadis fighting President Bashar al-Assad, a violent rivalry that has killed hundreds of fighters in recent months, rebels said.
Two rebels told Reuters that five ISIL members had entered Ahrar al-Sham headquarters in Aleppo, engaged its fighters and then one ISIL fighter blew himself up.
"Sheikh Abu Khaled was an important jihadi figure, he fought the Americans in Iraq and in Afghanistan. They (ISIL) gave the Americans a present, a free gift, by killing him," said a Syrian rebel close to the group.
"He was a very important commander, he is a close friend of Sheikh Ayman (al-Zawahiri) and he knew Sheikh Bin Laden."
Syrian rebels mourning al-Soury posted his picture on social media accounts. A fighter called for revenge saying that ISIL had "pushed it too far this time".
Al-Soury was born in Aleppo in 1963. A seior rebel source said he had been based in Afghanistan but was sent by Zawahiri to Syria a few months ago on a mission to try to end the infighting.
Sources said that, by killing al-Soury, ISIL had taken the war between jihadi factions to a new level, and that the decision to kill him must have been taken by the high command of ISIL, most probably its leader Abu Bakr al-Baghdadi, who fell out last year with Zawahiri and the leader of the Syrian al Qaeda group the al-Nusra Front.
"Sheikh Abu Khaled was influential among the jihadis, specially among the ISIL fighters. He was the only one who could make them change their minds and stop the fighting," another source said.
ISIL, which has attracted many foreign militants to its ranks, is a small but powerful force that emerged from the Sunni Islamist insurgency in neighbouring Iraq. In Syria, it has alienated many people by imposing harsh rulings against dissent in areas it controls.
Several Islamist rebel factions joined forces in January for an offensive to try to push their ISIL former allies out of rebel-held regions in northern and eastern Syria.
Sources close to Ahrar al-Sham said that al-Soury had rejected the infighting and opposed fighting ISIL. In an audiotape last year, Zawahiri appointed al-Soury as his man to mediate between jihadi groups to end the fighting.
The infighting has undermined the rebels' fight against Assad in the three-year-old civil war, and the prominence of hardline jihadi groups has made Western governments hesitant to back the rebellion. (Editing by Robin Pomeroy)