The inter-rebel war in Syria is heating up, after the leader of the Al-Qaeda-affiliated rebel group in Syria on Tuesday gave rival jihadists an ultimatum to accept arbitration by clerics or be expelled.

The BBC reported that Abu Mohammed al-Golani of the Al-Nusra Front warned the rival Islamic State of Iraq and the Levant (ISIS) that it would be driven from Syria and “even from Iraq” if it did not comply within five days.

The threat came after Sunday’s killing of an Al-Qaeda emissary, Abu Khaled al-Suri, at the hands of ISIS rebels.

Al-Suri, a commander from the Ahrar al-Sham group 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 carried out by ISIS members.

Al-Suri had reportedly been sent to end the clashes between ISIS and other rebels, reported the BBC.

The civil war in Syria has attracted many jihadist rebel groups which have been fighting the more moderate rebel groups in what has turned into a second war.

In recent weeks, the infighting between rebels has worsened, as three powerful rebel alliances – among them Islamist groups – have teamed up to fight ISIS, which they have warned is even worse than Assad’s regime.

ISIS has been accused of several human rights abuses, including torturing and murdering prisoners, among them children and teenagers, and forcing Druze men to convert to Islam or die.

In an audio message produced by Al-Nusra’s media arm and posted on jihadist websites on Tuesday, Al-Golani said Al-Suri had been a “man who solved problems, not one to create conflicts” and that they had seen each other a few days before his death.

“We say to his killers: may your hands perish and your deed be damned. You, those who give you your orders, and those who write your fatwas are wretched, O deceived ones,” he said, according to the BBC.

Without naming ISIS, Al-Golani denounced those attacking fellow rebels in Syria, using a derogatory term for President Bashar Al-Assad’s Alawite sect.