Twenty-three members of opposition factions were killed in clashes with Syrian regime forces in the northeastern region of Idlib.
The attack on a position held by the Jaish al-Izza faction took place on the edge of the northwestern province of Idlib, in an area due to be de-militarized under a Turkish-Russian agreement.
According to the Syrian Observatory for Human Rights, regime forces moved in to take a high building held by the opposition in a rural area of neighboring Hama province.
Idlib and some surrounding areas are the last major opposition bastion in Syria. The regime had threatened an assault on the territory, home to around three million people, but a deal for a de-militarized buffer zone around it was reached in September between Moscow and Ankara.
Several deadly skirmishes have occurred since the deal but 23 is the highest number of known fatalities in a single incident inside the planned buffer zone, the Observatory said.
"This is the highest death toll in the de-militarized zone since it was announced," Rami Abdel Rahman, head of the Britain-based monitoring group, told AFP.
He said at least 35 opposition fighters were also wounded in the clash but could not provide a casualty toll for regime forces.
It was not clear what prompted the attack, which did not appear to signal any large-scale regime offensive or otherwise threaten the September 17 truce deal.
The regime troops pulled out of the buffer zone after the flare-up, the Observatory said, adding that the fighting went on for much of the night.
Jaish al-Izza, which was formerly supported by the United States and is mostly active in the Lataminah area of Hama province, where the attack took place.
It is not a member of the main opposition alliance in the Idlib area and after initially rejecting the truce deal struck by Moscow and Ankara, it had begun complying and pulling back its heavy weaponry.
The Observatory said regime forces shelled parts of Jarjanaz, Khan Sheikhoun and other villages on Friday.
Only sporadic incidents have broken out in the 15- to 20-kilometer (nine- to 12-mile) buffer zone in the past two months, killing 18 civilians and three fighters before Friday's clash.
An array of opposition factions control Idlib, the dominant force being Tahrir al-Sham, an alliance led by fighters formerly linked to al-Qaeda.
This article has been adapted from its original source.
Copyright © Saudi Research and Publishing Co. All rights reserved.