AMMAN (Reuters) - Al Qaeda leader Ayman al-Zawahri urged rival jihadist fighters in Syria to unite or risk death but again decried fellow Sunni Muslim militants Islamic State as “extremists” in an audio recording posted online on Sunday.

Al Qaeda's Ayman al-Zawahri speaks from an unknown location, in this still image taken from video uploaded on a social media website June 8, 2011. REUTERS/Social Media Website via Reuters TV

As successor to Osama bin Laden, Zawahri 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 Islamic State, which controls territory in Syria and Iraq and followings in Libya and Yemen.

In Syria, al Qaeda offshoot Nusra Front and Islamic State 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.

“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,” Zawahri said.

The authenticity of the recording, the first since January, could not be immediately verified, but it had the hallmarks of previous Zawahri tapes. In January, the Egyptian former doctor called for revenge after Saudi Arabia executed dozens of militants.

He is believed to be hiding in a border area between Afghanistan and Pakistan.

On the recording, Zawahri 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 known as Jaish al Fateh, which is leading battles against Syrian government forces and its Russian- and Iranian-backed allies in the southern Aleppo countryside.

In January, Nusra Front tried unsuccessfully to convince rival Islamist factions to merge into one unit, including the powerful Ahrar al-Sham.

Zawahri also emphasized once again the ideological divide between al Qaeda and Islamic State, which is fighting a Western-led coalition and Russian forces while also clashing with Western-backed rebels and the Syrian army.

He described them as “extremists and renegades” whose followers would eventually disavow their beliefs and methods.