Clashes on Wednesday intensified between the two largest rebel alliances in northern Syria, killing more than 80, including civilians and fighters.
Battles between the Al Qaeda-linked Hay'at Tahrir Al Sham alliance and the Turkey-backed National Liberation Front in western Aleppo have spilled into adjacent provinces of Idlib and Hama, according to activists.
The skirmishes mark one of the most severe cases of rebel infighting in Syria's north since Hay'at Tahrir Al Sham launched a wide-scale offensive to seize most of Idlib province from rival rebel forces in 2017. They also effectively nullify an agreement reached between the Al Qaeda linked fighters and the rebels in October, during which both vowed to cease attacks.
Rebel infighting has also doused tensions on frontlines near the Kurdish-held town of Manbij as rebel forces in the area redeploy further west to support the National Liberation Front in its fight against Al Qaeda-linked fighters.
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Fighting in western Aleppo since Tuesday has killed at least 41 Hay'at Tahrir Al Sham militants and six civilians, the UK-based Syrian Observatory for Human Rights said on Thursday. The war monitor says 34 National Liberation Front fighters, mainly from the powerful Noureddine Al Zenki rebel group, have also died in clashes.
The casualty count is expected to climb if the battles continue.
Zenki spokesman Mohammad Adib says Hay'at Tahrir Al Sham are now expanding their attack and activating several frontlines.
The group has seized control of the strategic town of Darat Izza – one of the largest in western Aleppo province – following fierce battles with the Zenki group, according to the Observatory and Halab Today TV – an activist-run media outlet.
The militant group also made advances in southern and south-eastern Idlib on Thursday after launching an offensive at dawn, the Observatory and activists said.
Fighting is taking place in proximity to Turkish observation posts manned by Turkish troops, according to the war monitor.
The National Liberation Front released a statement on Wednesday demanding the mobilisation of all of its affiliates to counter attacks by Hay'at Tahrir Al Sham and reclaim territory seized by the group.
The Zenki spokesman said the increased mobilisation of rebel fighters is altering the power balance on the ground, aiding rebels in launching a counterattack to reclaim the territory they lost to Hay'at Tahrir Al Sham in recent days.
"The reality on the ground is different now after the general mobilisation was announced," Mr Adib said. "The NLF (National Liberation Front) is now shifting from a defensive strategy to absorb the HTS (Hay'at Tahrir Al Sham) attack towards an offensive strategy to counter militant gains."
The Observatory on Thursday said rebels regained control of territory they had lost to Hay'at Tahrir Al Sham earlier in the day.
Meanwhile late on Wednesday demonstrations broke out in rebel-held territory in Aleppo's countryside. Residents took to the streets to protest against what they described as Al Qaeda's aggression against rebels.
The battles between the rival rebel alliances come at a time when Ankara says it is planning to launch an operation against Kurdish groups in the city of Manbij, east of Aleppo.
Hundreds of Ankara-backed rebels who were positioned near the Kurdish-held city have now redeployed to areas in western Aleppo and southern Idlib where battles with Hay'at Tahrir Al Sham are gaining pace.
The Observatory on Thursday said that a sense of calm was prevailing over frontlines near Manbij following the withdrawal of rebel fighters.