Fighting erupted on Friday between Turkey-backed rebels and jihadist hardliners in northwest Syria, a monitor and a resident said, near a planned demilitarisation zone.
The buffer zone, agreed last month between rebel backer Ankara and government ally Moscow, is meant to separate regime fighters from the myriad rebel and jihadist forces of the Idlib region.
On Friday, just days away from the deadline to establish the zone, clashes erupted between rival rebels and jihadists.
The Syrian Observatory for Human Rights said fighting first began between Hayat Tahrir al-Sham (HTS), led by former Al-Qaeda fighters, and Nour al-Din Al-Zinki in the town of Kafr Halab.
HTS had reportedly been trying to arrest a local commander present in the town on the western edge of Aleppo province, near the administrative border with Idlib province.
"Zinki sent reinforcements to the area, and the clashes expanded to several areas and the National Liberation Front joined in," said Observatory head Rami Abdel Rahman.
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The NLF is the main Turkey-backed rebel alliance in the Idlib region, but jihadist heavyweight HTS holds most of the province.
"The clashes are near where the buffer zone is supposed to be established and are the biggest since the deal was announced last month," said Abdel Rahman.
The Britain-based Observatory said HTS had taken two towns, including Kafr Halab. At least three civilians, one HTS member, and two NLF fighters were killed, the monitor said.
"We woke up at 4:00am to the sound of gunfire from an HTS convoy," a resident of Kafr Halab told AFP.
"They started hitting civilian homes, and a child was killed," said the resident, speaking on condition of anonymity.
Russia and Turkey agreed last month to set up a U-shaped demilitarisation zone ringing Idlib that would be free of both heavy weapons and jihadists.
The NLF has cautiously welcomed the deal but HTS has yet to endorse it. Many of the agreement's details, including where exactly the zone would fall, remain murky.