BEIRUT — It only took a few days for Al Qaeda-linked militants to seize more than two dozen towns and villages in northern Syria from rival insurgents this month, cementing their control over an area the size of neighboring Lebanon.
The advance by Hayat Tahrir al-Sham, or the Levant Liberation Committee, was the most serious blow yet to a September cease-fire brokered by Russia and Turkey that averted a government offensive in Idlib province, the last main stronghold of the Syrian opposition.
It highlighted the growing threat posed by Al Qaeda at a time when its rival, the Islamic State group, is on the verge of defeat and the United States is preparing to withdraw its 2,000 troops from Syria. Although Hayat Tahrir al-Sham has formally severed ties with Al Qaeda, specialists say it is still closely linked to the global network founded by Osama bin Laden and could use its base in Syria to launch attacks in the West.
Fawaz Gerges, a professor of Middle Eastern politics at the London School of Economics, says there is a ‘‘real danger’’ that the group’s advance ‘‘will not only worsen the humanitarian crisis for the 3 million inhabitants there, but also give [Syrian President Bashar] Assad and his allies the justification to assault the province.’’
‘‘Such a scenario would be as devastatingly bloody as the battle for Aleppo,’’ he said, referring to the months of heavy fighting over Syria’s largest city in 2016, which killed thousands of people and ended with government forces and their allies capturing the rebel-held east.
Hayat Tahrir al-Sham includes many battle-hardened Al Qaeda fighters, and its capture of most of rebel-held Syria could force aid agencies to withdraw, leaving thousands of civilians to fend for themselves. The opposition’s Free Aleppo Medical Directorate said that some 250,000 people will lose medical support after 43 facilities it runs cease operations due to a drop in aid from Western agencies after the latest Hayat Tahrir al-Sham offensive.
The government has meanwhile stepped up its bombardment of Idlib and neighboring rebel-held areas. Pro-government media say Defense Minister General Ali Ayoub and Brigadier General Suheil al-Hassan have recently visited the front lines with Idlib, raising fears of a new government offensive.
Hayat Tahrir al-Sham now controls an area of about 3,475 square miles, or about 5 percent of Syria’s territory. The area is home to some 3 million people, many displaced from other parts of the country.
Turkey has nearly a dozen observation posts in Idlib, but has shifted its focus further east, where it is preparing to launch an offensive against Syrian Kurdish forces.
Ankara, which supports the opposition, fears the Syrian government is trying to undermine the September agreement.