The leader of al-Qaeda has urged warring fighters in Syria to unite or risk death while criticising again the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group in an audio recording.
In the clip, posted online on Sunday, Ayman al-Zawahiri criticised the UN-backed political process to find a solution in Syria and praised al-Nusra Front, an al-Qaeda offshoot which controls most of Idlib province.
"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 says.
Al-Nusra Front is part of an alliance of armed groups known as Jaish al-Fatah, which is leading battles against Syrian President Bashar al-Assad's forces and his Russian- and Iranian-backed allies in the southern Aleppo countryside.
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In January, al-Nusra Front tried unsuccessfully to convince rival Sunni factions - including the powerful Ahrar al-Sham - to merge into one unit.
As successor to Osama bin Laden, Zawahiri has the allegiance of al-Qaeda branches in the Middle East, North Africa and South Asia.
The authenticity of the recording, the first since January, could not be immediately verified, but it had the hallmarks of previous Zawahiri tapes.
He is believed to be hiding in a border area between Afghanistan and Pakistan.
'Extremists and renegades'
In the audio clip, Zawahiri emphasised once again the ideological divide between al-Qaeda and ISIL.
He described ISIL as "extremists and renegades" whose followers would eventually disavow their beliefs and methods.
Al-Qaeda's dominance is being challenged by ISIL, which controls territory in Syria and Iraq and has branches in Libya and Yemen.
|Al-Nusra and its allies have made gains in recent weeks in Aleppo at the expense of government forces [Reuters]
Once a single group in Syria, they split in 2013, largely owing to a power struggle among leaders.
The Syrian uprising, which started with largely peaceful protests in 2011 against the Assad government, has descended into a major conflict that has pulled in regional and global powers.
Violence has left at least 250,000 people dead and displaced half the country's pre-war 22 million population.
In the latest fighting, Syrian warplanes attacked rebels on Sunday near the northern city of Aleppo, reports said.
Dozens of air strikes hit near Khan Touman, a town south of Aleppo which the Jaish al-Fatah took from forces loyal to the government and Iran on Thursday, according to both fighters and Syrian state media.
Aleppo - one of the biggest strategic prizes in the war - has been carved up into government and rebel-held zones.
The surrounding region is also crossed by valuable supply routes into neighbouring Turkey.
Manar, the media outlet of the Lebanese Hezbollah group supporting Assad's troops in the area, said heavy fighting was going on against rebels.
Government forces have made significant advances in the northern region after Russia entered the war on their side in September.
Inside Aleppo, rebels said on Sunday that the Syrian army shelled and bombed their posts overnight near a frontline in the western part of the city near the Jamiyat al-Zahraa neighbourhood.
Rebels are seeking to take over the area that would allow them to enter the heart of government-held parts of Aleppo.
The UK-based Syrian Observatory for Human Rights said the rebels fired rockets on residential areas in government-controlled areas and in the main Saad al-Jabiri square, with reports of more casualties in the collapse of a building in Midan district which was hit by a missile.
In the western Aleppo countryside, in the rebel-held town of Kafrnaha, an air strike hit a hospital, killing several people, SOHR said.
For its part, Russia said the truce in Aleppo had been extended until Monday.
Separately, Amaq news agency, which is associated with ISIL, said the group had destroyed a gas plant in the desert outside the central city of Palmyra on Sunday.
ISIL fighters retreated from the ancient city two months ago but continue to operate in the surrounding area.