An al-Qaida-linked coalition in Syria has expanded its presence in the last major rebel stronghold in the country, cementing the extremists’ hold on the region in northwestern Syria.

Opposition activists say the al-Qaida-affiliated Hayat Tahrir al-Sham, Arabic for Levant Liberation Committee, or HTS, took over control of most of Idlib province and the surrounding countryside after forcing rival insurgents to accept a deal for a civil administration run by HTS in their areas.

The deal follows days of fighting during which the HTS seized more than two dozen villages from the Turkey-backed National Liberation Front.

The developments threaten to derail a cease-fire in the area reached in September between Turkey and Russia that averted a Syrian army assault on Idlib.