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, 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 also part of an alliance of armed groups known as Jaish al-Fatah, which is leading battles against Syrian government forces and its Russian- and Iranian-backed allies in the southern Aleppo countryside.
In January, al-Nusra Front tried unsuccessfully to convince rival factions to merge into one unit, including the powerful Ahrar al-Sham.
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As successor to Osama bin Laden, Zawahiri, an Egyptian former doctor, 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.
In the audio clip, Zawahiri emphasised once again the ideological divide between al-Qaeda and ISIL, which is fighting a Western-led coalition and Russian forces while also clashing with moderate rebels and the Syrian army of President Bashar al-Assad.
He described them 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 followings in Libya and Yemen.
In Syria, al-Nusra Front and ISIL 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.
Meanwhile, on the ground, Syrian warplanes attacked fighters near the northern city of Aleppo, both sides said, as the government tried to push back a rebel advance in the area.
Dozens of air strikes hit near the town of Khan Touman, which rebels took from forces loyal to the government and its ally Iran late on Thursday, fighters and Syrian state media reported.
Aleppo - one of the biggest strategic prizes in a war now in its sixth year - has been divided into government and rebel-held zones through much of the conflict.
OPINION: Al-Nusra Front's quest for a united Syria
The surrounding region is also crossed by valuable supply routes into neighbouring Turkey.
The Syrian army said it had hit what it described as terrorist groups hard on Sunday, but did not give details of any territorial gains.
Manar, the media outlet of the Iran-backed Hezbollah group supporting Syrian government forces in the area, said heavy fighting was going on against Sunni Muslim rebels.
Government forces had made significant advances in the northern region after their other main ally, Russia, entered the war in September.
But the seizure of Khan Touman on Thursday by the Jaish al-Fatah marked a major counterattack by forces opposed to Assad.
Iranian troop losses
The loss of the town south of Aleppo was a particular blow for Iranian troops who suffered one of their biggest single-day losses in the conflict.
Iran conceded on Saturday the loss of 13 soldiers near Aleppo in one of Iran's biggest single-day losses since it sent forces to support Assad.
Iran's Fars news agency quoted a Revolutionary Guards official as saying that 13 Iranian military advisers had been killed and 21 wounded.
The Syrian Observatory for Human Rights (SOHR), the UK-based monitoring group, said it had confirmed from its sources on the ground the death of 20 Iranians, including 13 advisers.
A fighter from al-Nusra Front - which is not involved in a shaky ceasefire across Syria - said on social media the group was now pushing further south towards the town of al-Hader, a stronghold for Hezbollah and Iranian forces.
Inside Aleppo, where Russia said it extended a truce that began on Wednesday until Monday, rebels said the Syrian army shelled and bombed overnight their posts 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.
SOHR said the rebels fired rockets on residential areas in government-controlled areas and in the main Saad al-Jabiri square injured seven, 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 with several killed, SOHR said.
Aleppo truce 'extended'
For its part, Russia said that a truce in Aleppo itself 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.
The Syrian uprising, which started with largely peaceful protests against the government, has descended into a major conflict that has pulled in regional and global powers.
It has at least 250,000 people and displaced half its pre-war 22 million population.
