* Nusra Front ultimatum is war declaration, sources say
* Comes two days after al Qaeda leader killed
* In-fighting between Islamist rebels to intensify (Adds details, quotes from ISIL, Nusra)
By Mariam Karouny and Dominic Evans
BEIRUT, Feb 25 (Reuters) - The head of al Qaeda's Syrian arm has given rival Islamist militants five days to accept mediation to end their infighting or face a war which will "eradicate" them, according to an audio recording posted on Tuesday.
Abu Mohammed al-Golani, leader of the Nusra Front, called on the Islamic State in Iraq and the Levant (ISIL) to agree to arbitration by religious scholars to end more than a year of feuding which has turned violent.
Golani's ultimatum comes two days after senior al Qaeda member Abu Khaled al-Soury was killed in a suicide attack in Syria. Nusra accused ISIL of killing him, a charge sources close to the splinter group have denied.
Sources on both sides said Golani's statement, the first since tensions between the two groups erupted, was a declaration of war.
"We will wait for five days from the date of this recording for your formal reply," Golani said in the audio tape, posted on an Islamist web site.
"By God, if you reject God's ruling again and do not put an end to your plague against the Umma (Muslim nation), then the Umma will launch an assault against this ignorant ideology and will eradicate it, even from Iraq," he 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.
Al-Soury, a friend of the late al Qaeda founder Osama bin Laden, had been appointed by the group's current leader, Ayman al-Zawahri, to mediate between Nusra and ISIL.
After the mediation failed, his Salafi group Ahrar al-Sham joined other Islamists brigades in rebel-held northern and eastern Syria to fight ISIL.
Hundreds of fighters have been killed in two months of infighting that has undermined the wider rebel campaign against President Bashar al-Assad.
Golani's Nusra Front, which has sworn allegiance to Zawahri, had avoided openly confronting ISIL, with which it shares a radical jihadi ideology, despite deep rivalry and tensions between the two groups.
Al Qaeda dissociated itself from ISIL after its leader al-Baghdadi, of Iraqi origin, rejected an order by Zawahri last year to dismantle the group and return to Iraq.
In his statement, Golani differentiated between ISIL and Western-backed groups the Supreme Military Command and the National Coalition of exiled political opponents of Assad, both of which he described as infidels.
Sources from Nusra said that the decision had already been made to fight ISIL.
"This ultimatum is just words. The decision has been taken. Nusra will fight," one of the sources told Reuters.
A source from ISIL denied the group's responsibility for the killing of Abu Khaled al-Soury and said his death had been used as an excuse to declare war against ISIL for the benefit of the United States.
"Sheikh Abu Khaled's killing was tragic to all of us. I tell you the State (ISIL) has nothing to do with it until our Emir al-Baghdadi says otherwise. And there is no evidence that the State did it," he told Reuters.
"Golani is doing a big favour to the Americans. His group is becoming a tool in the hands of Saudi intelligence too." (Editing by Robin Pomeroy)