AMMAN, May 8 (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.
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.
Related: The history of the fight against al-Qaida and the Taliban in Afghanistan:
History of fight against al-Qaeda, Taliban in Afghanistan
Al-Qaida chief tells jihadist fighters in Syria: Unite or die
ZHAWAR, AFGANISTAN - APRIL 1: Associated Press reporter and stringer Patrick O'Donnell walks up a riverbed outside of the Zhawar training camp built and controlled by Afghan mujahideen leader Jalaluddin Haqqani in the Afghan-Pakistan border area outside of Khost, Afghanistan in April 1,1990. Osama bin Laden also contributed heavy machinery and funds for construction of Zhawar training camp as well as the caves in the background that stored ammunition and heavy weapons for the Hezb-i-Islami (Khalis) mujahideen group Haqqani was aligned with. Haqqani befriended bin Laden and offered him land for training camps opened nearby for Arab fighters. Haqqani received funds from the CIA and the Pakistan's Inter Services Intelligence agency, or ISI. (Photo by Robert Nickelsberg/Getty Images)
UNSPECIFIED: Osama Bin Laden with Muhammad Atif, an ex-Egyptian police officer in 1997. Muhammad Atif has been killed by U.S. bombing in Southern Afghanistan. He was responsible for the killing of U.S. troops in Somalia in 1993 where he was commanding Al-Qaida troops. (Photo by MIR HAMID/DAILY DAWN/GAMMA/Gamma-Rapho via Getty Images)
AFGHANISTAN - MAY 26: (FILE PHOTO) (JAPAN OUT) (VIDEO CAPTURE) A man CNN identifies as Qaed Senyan al-Harthi (center, wearing red and white head-cloth) stands next to al Qaeda leader Osama Bin Laden (left in white turban) on May 26, 1998 in Afghanistan. According to CNN, Al-Harthi, also known as Abu Ali, was killed with six other men as they traveled in a truck in Yemen. CNN also reports that a U.S. 'Hellfire' missile fired from an unmanned CIA drone aircraft killed Al-Harthi, who was thought to be the main Al Qaeda operative in Yemen. (Photo by CNN via Getty Images)
AFGHANISTAN - AUGUST 20: (JAPAN OUT) (VIDEO CAPTURE) Osama Bin Laden, the Saudi millionaire and fugitive leader of the terrorist group al Qaeda, explains why he has declared a 'jihad' or holy war against the United States on August 20, 1998 from a cave hideout somewhere in Afghanistan. Bin Laden is thought to be the mastermind behind the September 11, 2001 terrorist attacks on New York and Washington D.C. (Photo by CNN via Getty Images)
SANGESAR - MARCH 1: Young Taliban attending a madrassa March 1, 2000 in Sangesar, Afghanistan congregate near the adobe mud apartment where Mullah Omar, the leader of the Taliban movement, stayed and studied during the Soviet occupation of Afghanistan. During a firefight against the the Soviet army, Mullah Omar was blinded by shrapnel from a Soviet mortar and lost the site in one eye. Mullah Omar and the ruling Taliban government were forced from power following the al Qaeda 9-11 attacks on the U.S.Mullah Omar is believed to be residing in neighboring Pakistan. (Photo by Robert Nickelsberg/Getty Images)
A video grab dated 19 June 2001 shows members of Saudi dissident Osama bin Laden's Al-Qaeda, or 'The Base',organization carrying AK-47 (Kalashnikov) sub-machine-guns in a video tape said to have been prepared and released by bin Laden himself. The United States was on alert on 23 June as reports came from the Middle East that bin Laden's fighters were preparing to hit US and Israeli interests around the world. Copies of the video tape, which shows him as well as Al-Qaeda guerrilla fighters training at their al-Farouq camp in Afghanistan, have been circulated to a limited number of Islamists. AFP PHOTO/Yasser Al-ZAYYAT (Photo credit should read /AFP/Getty Images)
395586 01: This undated Department of Defense photograph released October 9, 2001 shows the Qandahar surface-to-air missile site in Afghanistan before being attacked by US forces. Three days of airstrikes by US and British warplanes have begun to weaken the al-Qaida network and the Taliban regime, according to Pentagon officials. (Photo by Department of Defense/Getty Images)
395586 02: This undated Department of Defense photograph released October 9, 2001 shows damage to the Qandahar surface-to-air missile site in Afghanistan after being attacked by US forces. Three days of airstrikes by US and British warplanes have begun to weaken the al-Qaida network and the Taliban regime, according to Pentagon officials. (Photo by Department of Defense/Getty Images)
395586 07: This undated Department of Defense map released October 9, 2001 shows terrorist training camps, military sites and humanitarian aid sites in Afghanistan. Three days of airstrikes by US and British warplanes have begun to weaken the al-Qaida network and the Taliban regime, according to Pentagon officials. (Photo by Department of Defense/Getty Images)
395589 04: (FILE PHOTO) Children participate in military training in this undated still frame from a recruitment video for Osama bin Laden''s extremist Al-Qaida network. (Photo by Al Rai Al Aam/Feature Story News/Getty Images)
397425 13: A collection of documents lay on the floor of a room inside a house on a suspected Al Qaeda base November 16, 2001 in Kabul, Afghanistan. Items found include a page torn from a flying magazine with flight training ads, a map of Afghanistan, packaging from a copy of Microsoft Flight Simulator 98, detailed notes writted both in Arabic and English and other documents. (Photo by Tyler Hicks/Getty Images)
397428 08: A bomb making notbook is shown November 17, 2001 in an abandoned house once used by Osama bin Laden's Al-Qaeda terrorist network in Kabul, Afghanistan. The residential house is littered with wires and all sorts of electronic and other bomb-making gear and was abandoned when Northern Alliance troops approached the city before it's fall to the rebels. (Photo by Scott Peterson/Getty Images)
398388 01: Former Taliban fighters, who surrendered to the Northern Alliance following their defeat in Kunduz a week ago, gather at a window to the courtyard of a jail complex December 8, 2001 in Shebargan, Afghanistan. Although the prisoners have not been sentenced, about a third of them are suspected of being members of Saudi dissident Osama Bin Laden's al Qaeda terrorist group. (Photo by Oleg Nikishin/Getty Images)
KANDAHAR, AFGHANISTAN - JANUARY 1: Two CH-46 Sea Knights land at the Marine's base at Kandahar International Airport, 01 January, 2002. U.S. Marines and Afghan allied forces executed a raid on an as-yet unnamed location in the hopes of finding Taliban and al-Qaida forces. (Photo credit should read ROB CURTIS/AFP/Getty Images)
400454 03: This Osama Bin Laden propaganda poster was found in an al Qaeda classroom by U.S. Navy SEALs on a search and destroy mission in the Zhawar Kili area January 14, 2002 in Eastern Afghanistan. Navy special operations forces are conducting missions in Afghanistan in support of Operation Enduring Freedom. (Photo by U.S. Navy/Getty Images)
400160 05: An FBI wanted poster presented by US Attorney General John Ashcroft shows Al Rauf Bin Al Habib Bin Yousef Al-Jiddi (top and bottom ) and Faker Boussora (bottom R) January 25, 2002 in Washington, DC. Ashcroft said that Al Rauf Bin Al Habib Bin Yousef Al-Jiddi, 36, a Canadian citizen born in Tunisia, was identified in part from a suicide letter found in the rubble of the Afghanistan residence of Mohammad Atef, believed to have been Osama bin Ladens military chief. (Photo by FBI/Getty Images)
401308 15: U.S. Navy SEALs explore the entrance to one of 70 caves they discovered during a Sensitive Site Exploitation (SSE) mission January 14, 2002 in Zhawar Kili area of Eastern Afghanistan. The caves used by al Qaeda and Taliban forces were destroyed by Navy Explosive Ordnance Disposal (EOD) personnel or air strikes called in by the SEALs. The Navy SEALs are in Afghanistan supporting Operation Enduring Freedom. (Photo by U.S. Navy/Getty Images)
402085 12: United States Army 10th Mountain soldier Jorge Avino from Miami, Florida carves the body count that their mortar team has chalked up on a rock March 9, 2002 near the villages of Sherkhankheyl, Marzak and Bobelkiel, Afghanistan. The team said they have killed 40 plus people, hit 12 vehicles and destroyed 1 mortar team near the villages were an al Qaeda and Taliban stronghold came under intense bombing and firefights as the coalition forces battled to root them out. (Photo by Joe Raedle/Getty Images)
SHAH-E-KOT, AFGHANISTAN - MARCH 17: United States Army and Canadian soldiers look over the rugged Shah-e-Kot mountains as they search for caves or Taliban and al-Qaida fighters on the loose, 25 km (15 miles) southeast of Gardez, in Afghanistan, 15 March 2002. Hundreds of American and Canadian troops were lifted into the mountainous region at high altitude to search for and destroy any enemy they encounter. (AP Photo/ Mikhail Metzel, Pool) (Photo credit should read MIKHAIL METZEL/AFP/Getty Images)
HESARK, AFGHANISTAN - JULY 16: U.S. Army soldiers from the 101st Airborne watch from defensive positions as an inbound Blackhawk helecopter lands July 16, 2002 in village of Hesarak, eastern Afghanistan. The army raided the village to conduct a follow-up search for possible Al-Qaida or Taliban intelligence materials and to provide humanitarian aid. The raid was a follow-up to a similar raid there four days ago that yielded two detainees and undisclosed intelligence materials. (Photo by Scott Nelson / Getty Images)
QIQAY, AFGHANISTAN -JULY 29: Two Afghan boys sit together in a village that is suffering from a shortage of food and water July 29, 2002 in Qiqay, Afghanistan. As U.S. soldiers continue their search for hidden weapons and equipment left by retreating Taliban and al-Qaeda forces, they try to assess the needs of Afghans and pass on their information to humanitarian organizations. (Photo by Wally Santana/Pool/Getty Images)
KABUL, AFGHANISTAN - SEPTEMBER 14: Afghan soldiers wait to train for the Afghan National Army (ANA) at a base September 14, 2002 outside of Kabul, Afghanistan. At the request of the Afghanistan Interim authority, U.S. military forces and French Army forces are training, advising and assisting the ANA at the compound which was once used as a training site by the Mujahadeen, Taliban and al Qaeda fighters. (Photo by Ami Vitale/Getty Images)
KABUL, AFGHANISTAN - MARCH 2: US Army spokesman Lt. Col. Bryan Hilferty speaks to the media at a press centre on March 2, 2004 at in Kabul, Afghanistan. The foreign media has returned to Afghanistan following reports that the capture of Osama Bin Laden is nearing, although Lt. Col. Hilferty deflected questions regarding those reports. (Photo by Darren McCollester/Getty Images)
KABUL, AFGHANISTAN - OCTOBER 9: (US NEWS AND WORLD REPORT AND NEWSWEEK OUT) Afghans citizens in Kabul, Afghanistan, October 9, 2004 vote at the Jaffaria Mosque in their first ever presidential elections. Up to 10.5 million Afghans were registered to vote. The vote was marred by accusations of voting irregularities with 14 candidates boycotting the results. (Photo by Robert Nickelsberg/Getty Images)
-, -: A video grab taken 06 July 2006 from the pan-Arab satellite television network al-Jazeera shows al-Qaeda second-in-command Ayman al-Zawahri. In this video produced by the al-Qaeda linked media group Assahab, al-Zawahri claimed, on the eve of the anniversary of the July 7, 2005 London bombings, that a string of attacks will continue and become stronger until forces were pulled out of Afghanistan and Iraq and until financial and military support to America and Israel ended. **QATAR AND INTERNET OUT** (Photo credit should read -/AFP/Getty Images)
IN FLIGHT, PAKISTAN - FEBRUARY 17: Pakistan's North Waziristan tribal area is seen from the air February 17, 2007. NATO and the Afghan government say that Taliban and Al-Qaeda fighters cross into Afghanistan from the Pakistani side to stage attacks on NATO troops, especially since a peace deal was signed in September 2006 between Pakistani forces and the Taliban. The Pak military, which has some 80,000 troops stationed in the tribal areas, says the Afghan government and NATO should stop blaming Pakistan for Afghanistan's internal problems. (Photo by John Moore/Getty Images)
URSTAN PASS, AFGHANISTAN - MAY 2007: Mira Wars and his fellow soldiers from the Afghan National Army, Weapons Company, 2Â¼ Kandak 3 Brigade 201 Corp., based in Gowardehs OP (observations post) 3 miles from the border with Pakistan (Nuristan Region) take a break and pray. This company has soldiers from all the different tribes living in Afghanistan. (Photo by Alvaro Ybarra Zavala/Edit by Getty Images)
MAIMANA, AFGHANISTAN - DECEMBER 2007: Ahmadullah Rais, 40, is a high-ranking commander under the orders of General Dostum, and is awaiting the general's order to take up the fights against the Taliban once more, in Maimana, Afghanistan. (Photo by Veronique de Viguerie/Edit by Getty Images)
LAGHMAN PROVINCE, AFGHANISTAN - AUGUST 23, 2008: A map showing Spir Kundey, where on August 18, 2008, French soldiers on a patrol in Uzbeen Valley were ambushed by insurgents. 10 French soldiers were killed as well as one of their translators. Commander Farooki who is part of the Emirati Islami gathering, the Taliban, the Hezb-i-Islami, the Tora Bora front, and Al Qaeda took part in the ambush with his men. His men took two Famas (French weapons) from the dead bodies of the soldiers. (Photo by Veronique de Viguerie/Getty Images)
WATAPOR VALLEY, AFGHANISTAN - SEPTEMBER 4: US Army soldiers from the 4th Infantry Division's 4th Brigade, 2nd Battalion - 12th Infantry Regiment, based in Fort Carson, Colorado, fly through mountainous terrain along the Watapor Valley September 4, 2009 in Kunar province, Afghanistan. Local Afghan taliban and foreign fighters have strongly resisted the presence of US forces in Kunar. (Photo by Robert Nickelsberg/Getty Images)
MATTANI, NORTH-WEST FRONTIER PROVINCE, PAKISTAN - DECEMBER 22, 2009: New recruits being trained under the authority of Abdul Rehman, who claimed they were part of the Lashkar-e-Taiba Islamist militant group, however this claim was later denied upon verification by senior contacts within that group. Part of their training program was a 15km hike with weapons training. The best recruits will be sent on to a commando training camp in Kashmir, before taking up jihad in Afghanistan, India or Iraq. Lashkar-e-Taiba is a Pakistani jihadist group created in the 1980s to fight in Afghanistan, and especially in Indian Kashmir. Their members have often claimed they were trained by former military officers. They were also allegedly employed by the Pakistani intelligence agency called the Inter-Services Intelligence (ISI) in India and Afghanistan. They were officially banned in 2001, following an attack on parliament in New Delhi, India, so they regrouped under the name Jama'at-ud-Da'wah. It is widely claimed that the group was responsible for the Mumbai bombings in November 2008, which killed nearly 200 people. Since then, they have been on the UN list of terrorist organizations. The US authorities liken them to al-Qaeda, and as much of a security risk. The Pakistani authorities in Islamabad state that they have dismantled Lashkar-e-Taiba, however overseas intelligence services state this is not possible. The group's fighters are implicated in conflicts in Afghanistan, Iraq, and India, however they are not currently in battle with the Pakistani authorities, due to a supposed good relationship with the military there. They fight alongside the Taliban against NATO forces, and are the best trained and most battle-hardened fighters in the region. Their training is very military and regimented, compared to that of the Taliban which is more basic. (Photo by Veronique de Viguerie/Getty Images)
Four of the twelve Al-Qaeda suspects are seen behind bars during their hearing in a special court in the Yemeni southeastern port of Mukalla, charged with terrorism-linked offences on October 11, 2010. The defendants are charged with forming an armed group, planning attacks and procuring passports with the intention of using them to join Al-Qaeda in Afghanistan, Iraq and Somalia. AFP PHOTO/STR (Photo credit should read STR/AFP/Getty Images)
An Afghan man reads a newspaper on a street of Kabul on May 3, 2011, which details the death of Al-Qaeda leader Osama bin Laden. The United States has warned that it would probe how Osama bin Laden managed to live in undetected luxury in Pakistan, as gripping details emerged about the US commando raid that killed the Al-Qaeda kingpin. Officials said DNA tests had proven conclusively that the man shot dead by US special forces in Abbottabad was indeed the Islamist terror mastermind who boasted about the deaths of 3,000 people in the September 11 attacks of 2001. AFP PHOTO/Massoud HOSSAINI (Photo credit should read MASSOUD HOSSAINI/AFP/Getty Images)
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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.