An al-Qaeda breakaway group says it faces a war with rival Islamic factions who want to destroy it, describing Syria's al-Qaeda-linked branch as the "front of betrayal and treason".

The statement by the Islamic State of Iraq and the Levant (ISIL) on Friday came a week after the leader of the al-Qaeda-linked Jabhat al-Nusra gave it a five-day ultimatum to accept mediation to end infighting or be "expelled'' from the region.

Battles between the rival groups that began in early January have killed more than 3,000 people, activists say. The fighting in northern Syria, in which Nusra is backed by other Islamic groups, has been the deadliest between opposition fighters since the country's crisis began three years ago.

ISIL's spokesman, Abu Mohammed al-Adnani, said in a 22-minute audio clip posted online that his group's rivals tried to defeat ISIL "when they saw it was getting more powerful''.

"They resorted to force and thought that they would be able to wipe it within a few days or hours. They prepared for a military campaign all over Syria and accompanied it with a propaganda campaign to harm the image of [ISIL] and turn the people against it,'' Adnani said.

'Betrayal and treason'

"They then launched the campaign of betrayal and treason but they were surprised that [ISIL] was more solid than they expected and they failed in breaking it."

Last week, Abu Mohammed al-Golani, Nusra's leader, suggested arbitration by Muslim leaders to stop the infighting. He warned ISIL that it would be driven from Syria and "even from Iraq'' if it rejected the results of arbitration.

Adnani responded that his group did not reject offers to resort to arbitration but he criticised Nusra's top Muslim cleric and a member of its Shura Council.

Meanwhile on Friday, the British-based Syrian Observatory for Human Rights reported heavy fighting between ISIL and Nusra in the northeastern town of Markada, saying 23 fighters were killed on both sides.

The observatory said ISIL fighters had advanced in the town and captured wheat silos in the area.