Turkey and Russia have been unable to rein an al-Qaeda-linked group in Syria that has emerged as the terrorist organization’s strongest branch and the most dominant jihadi group in the country, Fox News reported Wednesday.

Although they are on opposite sides of the war, pro-Syrian opposition Turkey and dictator Bashar al-Assad’s ally Russia reached an agreement late last year to establish a horseshoe-shaped buffer zone in Idlib free of jihadis and their heavy weapons by October, to no avail. The Russia-Turkey pact is part of mutual efforts to end the war in Syria — raging since March 2011.

Known as Hayat Tahrir al-Sham (HTS), the al-Qaeda-linked group continues to wreak havoc in Idlib, the jihadis organization’s primary stronghold in the region.

“HTS has proven itself to be the strongest military force in the Idlib pocket in recent weeks. Their rapid advance is a cause for concern because they are de-stabilizing efforts to bring a political settlement to the last rebel enclave. Their extremist ideology also makes it more likely that the Russian and Assadist forces will have an excuse to launch a full-scale offensive, which will have devastating humanitarian consequences,” Alan Mendoza, founder and executive director of the U.S. policy think tank the Henry Jackson Society, told Fox News.

In recent days, the al-Qaeda affiliates conquered the strategic town of Darat Izza, located close to two main highways in Syria used to move fighters and supplies within the country and along the Turkish border.

Fox News noted:

An Al Qaeda-linked jihadist group Hay’at Tahir al-Sham (HTS) is gaining swaths of territory in the northern province of Idlib, usurping control from Turkish-backed groups connected to the National Liberation Front (NLF). The goals of the group, otherwise known as the “Organization for the Liberation of the Levant,” are centered on dominating as much territory as possible ahead of planned talks between Russia and Turkey, to bring a final end to the almost eight-year Syrian civil war. Based on a previous agreement, Turkey was supposed to have reigned in the extremist outfit – now raising concerns it may not have the capacity to do so.

Citing several analysts, Fox News noted that Turkish President Recep Tayyip Erdogan’s near single-minded focus on pushing the U.S.-allied Syrian Kurds out of territory bordering Turkey had rendered Ankara incapable of addressing the al-Qaeda threat in Idlib.

According to an analysis by the Center for Strategic and International Studies (CSIS), HTS has between 12,000 and 15,000 fighters and is the most dominant of all rebel groups operating in Syria.

Formerly known as al-Nusra Front and Jabhat Fateh al-Sham, HTS claims to have cut its allegiance with al-Qaeda, but several experts have dismissed the claim as deceptive.

The U.S. Department of State has officially designated HTS a foreign terrorist organization in May 2018.

“If HTS acts as a spoiler to the [Russia-Turkey] agreement on the ground, this will probably lead to one of two scenarios: either Turkey and the NLF launch military action against HTS, or Russia will seize the opportunity with the support of the [Assad] regime and its allies to enter Idlib,” Nawar Oliver, an analyst for the Turkey-based Omran Center predicted in October, according to Al Jazeera.

“The ramifications of that move could be vast,” he added.