Syria's official Al-Qaeda affiliate on Tuesday gave rival jihadists a five-day ultimatum to submit to a joint Islamic court after a top operative was killed in a suicide bombing.
The ultimatum issued by Al-Nusra Front to the Islamic State of Iraq and the Levant (ISIL) comes after the killing of Abu Khaled al-Suri, the commander of an Islamist brigade who was close to Al-Qaeda chief Ayman al-Zawahiri.
Rival rebels accuse ISIL of being behind the fatal attack and Al-Nusra's chief said his outfit would fight the group in Syria and neighbouring Iraq if it refused joint arbitration.
Though both are rooted in Al-Qaeda in Iraq, relations between Al-Nusra and ISIL have collapsed in recent weeks as Syria's rebels have turned on ISIL, accusing it of kidnapping, torturing and killing activists and rival rebels.
Even many hardline Islamist rebels have been critical of ISIL's quest for hegemony and its refusal to submit to joint arbitration in Islamic courts not run by ISIL clerics.
"We are waiting for you to respond within five days of the release of this recording," Nusra chief Abu Mohammad al-Jolani said in an audio file linked to the group's official Twitter account.
"Should you refuse... hundreds of members of the Ummah (Islamic nation) will fight (your) ignorant and aggressive ideology, even in Iraq," he said.
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The call for ISIL to submit to an Islamic court did not refer to a trial for Suri's killers, but was instead a request for ISIL to accept rulings and decisions of non-ISIL clerics.
ISIL has never heeded the rulings of clerics from outside its ranks.
ISIL first appeared in Syria in late spring last year, and its battle-hardened fighters were initially welcomed as allies fighting to overthrow President Bashar al-Assad.
But in January, rebels declared war against its jihadists, citing its abuses and refusal to cooperate with other groups.
Al-Nusra Front initially sought neutrality, but in recent weeks launched a war of its own against ISIL in the oil-rich eastern province of Deir Ezzor, a key conduit for weapons and fighters from neighbouring Iraq.
Relations between the two jihadist groups have hit a new low since Sunday's killing in a suicide bombing of Suri, a top Al-Qaeda operative and commander of Ahrar al-Sham, a powerful rebel group.
Rebels have accused ISIL of killing Suri, who has been hailed on jihadist websites as a companion of Zawahiri and Al-Qaeda's late founder Osama bin Laden.
Earlier this month, Al-Qaeda announced "it is not linked" to ISIL after the group ignored an earlier order from Zawahiri to return to Iraq.
Fighting between ISIL and its rivals has killed hundreds of people, mostly fighters on both sides, since early January.