The Al-Qaeda-linked Islamic State of Iraq and the Levant (ISIL) has reached out to other rebel groups in Syria to head off infighting, in an audio message posted online Sunday.

"Today, the (Islamic) state is reaching out to you to stop fighting us, to focus on fighting the nusairiyah," the voice in the message, purportedly that of ISIL chief Abu Bakr al-Baghdadi, said, using the term often used by jihadists when referring to the Syrian regime.

The remarks mark a shift for ISIL, which said earlier this month it would "crush" opposition fighters and considered members of the Syrian National Coalition and the military command of the Free Syrian Army to be "legitimate targets".

Baghdadi, in the audio message posted on jihadist forums, accused the rebels groups of "betrayal".

"Anyone who fought against us should review his stance," he said. "You have stabbed us in the back, as all of our soldiers were on the frontline."

He continued, referring to various parts of Syria, "We used to have an army who implemented a plan to invade Aleppo and advance to the western front, and we used to have forces in Hama and we used to have forces in Idlib.

"But all of this has stopped in one night because of the betrayal."