Islamist fighters advanced Friday against insurgents in Syria’s last major opposition stronghold after four days of clashes that have killed more than 100 combatants, a UK-based war monitor said.

Hayat Tahrir al-Sham, a jihadist-led alliance, captured more than 20 towns and villages from rival insurgents in the northern province of Aleppo, according to the Syrian Observatory for Human Rights.

“HTS was able to take control of areas previously held by Nureddine al-Zinki in the west of the Aleppo countryside,” the monitor said.

Those areas lie in the northeast of Syria’s last major insurgent stronghold, which includes a large part of Idlib province as well as adjacent parts of Aleppo and Hama provinces.

The clashes broke out after HTS, which is led by Syria’s former Al-Qaeda affiliate, on Monday accused Nureddine al-Zinki of killing five of its fighters.

Fighting erupted on Tuesday in the northern province of Aleppo before spreading to the provinces of Idlib and Hama in the following days.

Two HTS fighters and 14 insurgents were killed in the fighting on Friday, the Observatory said.

That brings the overall death toll to 61 HTS fighters, 58 insurgents, and eight civilians in four days, it said.

Over the past two years, HTS has regularly fought a rival alliance of insurgents called the National Liberation Front, which includes Nureddine al-Zinki, for territory in and around Idlib province.

– with reporting by Agence France-Presse