* Some 1,400 have died in infighting -monitoring group
* Internecine clashes have gone on for about 20 days
* Government and opposition meeting at peace talks in Geneva (Adds rebel infighting in Syria)
BEIRUT/ABU DHABI, Jan 23 (Reuters) - Al Qaeda leader Ayman al-Zawahri called on rebels in Syria to stop fighting each other as a faction linked to his group pushed rival insurgents from a northern town on Thursday.
The small but powerful al Qaeda-linked Islamic State in Iraq and the Levant (ISIL) has been caught up in clashes with other insurgents in Syria in recent weeks, often triggered by disputes over authority and territory.
About 1,400 people have died in the infighting over the last 20 days, the Britain-based Syrian Observatory for Human Rights said on Thursday. Some 190 of them were civilians caught in the crossfire or executed, it said.
The internecine fighting is the worst to break out during the nearly three-year revolt against President Bashar al-Assad and has further divided an already fractured opposition.
Zawahri urged the rebels to set up a committee to sort out their differences, according to an audio recording released on Islamist websites.
"Our hearts and the hearts of the (Muslim) nation, which hangs its hopes on you, have bled for the infighting that has spread between the ranks of those waging jihad for Islam," Zawahri said in the five-minute recording.
"We call on all our brothers in all the jihadist groups ... to work towards ending this sedition, which will lead to only God knows what," Zawahri said.
Reuters was unable independently to confirm the authenticity of the recording, but the voice bore a clear resemblance to that of Zawahri.
ISIL GAINS GROUND
In northern Syria, where hardline Islamists have taken stretches of territory from government forces, ISIL fighters took full control of the town of Manbij, northeast of the commercial hub of Aleppo, the Observatory said.
The pro-opposition Observatory, which monitors developments in Syria through a network of activist, military and medical sources, said at least nine non-ISIL rebels were killed in the clashes, as well as a number of ISIL fighters.
The insurgent groups fighting ISIL range from relatively secular moderates to hardline Islamists, including the al Qaeda-aligned Nusra Front.
In April, the head of ISIL, Abu Bakr al-Baghdadi tried to merge his group with the Nusra Front, defying orders from Zawahri and causing a rift.
The announcement angered the Nusra Front, which said it had not been informed, prompting Zawahri to step in, telling both to stop arguing and calling off the attempted merger.
The fissures have depleted rebel ranks and helped Assad's forces claw back territory around Aleppo.
They have also added a new layer of complexity to a conflict that has killed well over 100,000 people as government and opposition representatives meet for the first time at United Nations-brokered peace talks in Switzerland.
Syria's conflict began with peaceful protests against 40 years of Assad family rule and has descended into a sectarian conflict with the opposing sides armed and funded by Sunni Gulf Arab states and Shi'ite Iran. (Reporting by Ahmed Tolba and Alexander Dziadosz; Writing by Maha El Dahan and Alexander Dziadosz; Editing by Robin Pomeroy)
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