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May 9, 2016, 9:42 AM UTC / Updated May 9, 2016, 9:46 AM UTC / Source : Reuters

AMMAN, Jordan — Al Qaeda leader Ayman al-Zawahiri urged rival jihadist fighters in Syria to unite or risk death but again decried fellow Sunni Muslim group ISIS as "extremists" in an audio recording posted online on Sunday.

Al Qaeda leader Ayman al-Zawahri in 2014. SITE via Reuters TV

As successor to Osama bin Laden, Zawahiri has the allegiance of al Qaeda branches in the Middle East, North Africa and South Asia. But the group's dominance is being challenged by ISIS, which controls territory in Syria and Iraq.

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In Syria, al Qaeda offshoot Nusra Front and ISIS are the two most powerful groups fighting government forces. Once a single group, they split in 2013, largely due to a power struggle among leaders.

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"We have to want the unity of the Mujahideen in Sham [Syria] so it will be liberated from the Russians and Western crusaders. My brothers... the matter of unity is a matter of life or death for you," Zawahiri said.

The authenticity of the recording, the first since January, could not be immediately verified, but it had the hallmarks of previous Zawahiri tapes. Zawahiri is believed to be hiding in a border area between Afghanistan and Pakistan.

Related: ISIS Is Winning the Online War With Al Qaeda

On the recording, Zawahiri lambasted a U.N.-backed political process to find a solution in Syria and praised Nusra Front, which controls most of Idlib province. Nusra is also part of an alliance of Islamist brigades leading battles against Syrian government forces in the southern Aleppo countryside.

Zawahiri also emphasized once again the ideological divide between al Qaeda and ISIS, describing ISIS as "extremists and renegades" whose followers would eventually disavow their beliefs and methods.