The Al Qaeda-affiliated Al Nusra Front have sent an ultimatum to the rival Islamic State of Iraq and Syria (ISIS) to cease killing fellow rebels or face “expulsion” from Syria.

The message, sent by Abu Mohammed Al Julani, leader of the Al Nusra Front, said that ISIS would be driven from Syria if it did not comply within five days.

Posted on a number of jihadist websites, the message condemned the killing of rebel leader Abu Khaled Al Suri in a suicide bomb attack on Sunday. Julani said Suri had been a “man who solved problems, not one to create conflicts,” and added: “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.”

ISIS have waged a bloody campaign against fellow rebels, whom they regard as being insufficiently devout, and are widely suspected to be responsible for the killing of Suri. They have become known for the brutal enforcement of their extreme interpretation of Islamic law in the territory they control.

While Julani admitted that some Syrian rebel groups were guilty of “takfir” (unbelief), he accused ISIS of indiscriminately killing rebels and labelling them all as takfir.

He went on to demand that ISIS cease all killing and allow a group of senior clerics to rule on allegations of takfir.

Infighting between the Syrian rebel groups has intensified in recent months, with ISIS and Al Nusra severing links and fighting one another for territory. While Al Nusra have tried to build links with other rebel groups, including more secular ones, ISIS have focused on religious purity and stamping out more moderate voices.