The leader of a powerful Al Qaeda-linked group in Syria gave a rival breakaway group a five-day ultimatum to accept mediation by leading preachers to end infighting or be “expelled” from the region.
The ultimatum, announced in an audio recording by the leader of the Nusra Front, aims to end months of deadly violence between the Islamic State of Iraq and Greater Syria (ISIS)  and other Islamist factions. The fighting has killed hundreds of people since the beginning of the year and is undermining their wider struggle against President Bashar Assad.
It comes two days after the killing of Abu Khaled Al Suri, who had acted as Al Qaeda chief Ayman Al Zawahri’s representative in Syria. Nusra accused ISIS of killing him , a charge sources close to the splinter group have denied.
Both the Nusra Front and ISIS are considered terrorist organizations by the United States.
Zawahri  has named the Nusra Front Al Qaeda’s branch in Syria and broken ties with ISIS, which has increasingly clashed with rebel brigades in opposition-held areas of Syria. ISIS has angered other factions with its brutal tactics and campaign to Islamize areas under its control in the northeast.
More than 2,000 people have been killed in the fighting between ISIS and rebel groups, including the Nusra Front.
Abu Mohammad Al Golani, the Nusra Front leader, suggested in the audio recording arbitration by preachers to stop the infighting . He warned ISIS that it would be driven from Syria and “even from Iraq” if it rejected the results of arbitration. He did not elaborate on how his group might do that.
Sources on both sides said Golani’s statement, the first since tensions between the two groups erupted, was a declaration of war.
“We are waiting for your official answer within five days of issuing this statement,” Golani said in the audio message posted on militant websites. “By God, if you reject God’s judgment again, and do not stop your arrogant overlording over the Muslim nation, then [we] will be forced to launch an assault against this aggressive, ignorant ideology and will expel it, even from Iraq.”
Golani suggested the arbitration be conducted by three senior Al Qaeda ideologists, including two imprisoned in Jordan and one imprisoned in Saudi Arabia. He did not say how they would handle the arbitration while they are in detention.
The call for ISIS to submit to an Islamic court did not refer to a trial for Suri’s killers, but was instead a request for ISIS to accept rulings and decisions of non-ISIS preachers.
ISIS has never heeded the rulings of preachers from outside its ranks.
Suri, a friend of the late Al Qaeda founder Osama bin Laden, had been appointed by Zawahri to mediate between Nusra and ISIS.
After the mediation failed, his Salafist group Ahrar Al Sham joined other Islamist brigades in Syria’s rebel-held north and east to fight ISIS.
Golani’s Nusra Front, which has sworn allegiance to Zawahri, had avoided openly confronting ISIS, with which it shares a radical jihadist ideology, despite deep rivalry and tensions between the two groups.
Al Qaeda disassociated itself from ISIS after its leader Abu Bakr 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 ISIS 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 the decision had already been made to fight ISIS. “This ultimatum is just words. The decision has been taken. Nusra will fight,” one of the sources told Reuters.
A source from ISIS denied the group’s responsibility for the killing of Suri and said his death had been used as an excuse to declare war against ISIS for the benefit of the United States.
“Sheikh Abu Khaled’s killing was tragic to all of us. I tell you the State [ISIS] has nothing to do with it until our Emir Baghdadi says otherwise. And there is no evidence that the State did it,” he told Reuters.
“Golani is doing a big favor to the Americans. His group is becoming a tool in the hands of Saudi intelligence too.”
In related news from Syria, the Observatory reported that the Syrian government troops made advances Tuesday against rebel positions near the city of Aleppo, while rebels claimed they were holding their ground in the Qalamoun region near the border with Lebanon.
In Aleppo,  the Observatory said that government troops, backed by National Defense paramilitary forces and officers from Lebanon’s Hezbollah, advanced against rebel groups near Aleppo’s airport, and close to the Army’s 80th Division headquarters.
At least 17 rebels, including some non-Syrians, were killed in the fighting, the Observatory said.
It said that regime troops were now one kilometer away from the neighborhood of Tariq Al Bab, which has been targeted repeatedly by regime airstrikes and barrel bombs in recent weeks.
The Observatory said the government troops, backed by paramilitary forces, seized a factory in the Sheikh Said area of Aleppo while rebels, backed by the jihadist Nusra Front, disabled a regime vehicle in Sheikh Najjar, adding that the regime side sustained casualties in the clashes.
Three groups – the Nusra Front, the Mujahedeen Army and the Islamic Front alliance of seven militias – have announced that they were joining forces to set up a unified command center to help fight back against the government’s latest efforts to advance on the northern city. 
Despite the calls for unity in the face of the regime offensive, the Observatory reported that two rebel battalions clashed in the city, with two civilians killed in the crossfire.