The Al Qaeda chief has reportedly urged members of the group's Syrian branch, the al Nusra Front to put an end to the ongoing fighting against their jihadist rivals.
Ayman al Zawahiri ordered all soldiers of the front immediately cease fighting one another in an audiotape posted online.
According to Sky News, al Nusra's leader Abu Mohammed al Jolani was told to "devote himself to combat the enemies of Islam, specifically Baathists, Shiites and their allies".
Al Nusra has been locked in fierce fighting with the Islamic State of Iraq and the Levant (ISIL) since the start of the year.
He said ISIL's presence had hindered the fighting against forces loyal to the Syrian president because it had created divisions which had proved to be "a gift to Assad," the report added.
Zawahiri repeatedly ordered ISIL leader Abu Bakr al Baghdadi to devote himself to wounded Iraq, which required the group to redouble its efforts.