While the U.S.-led fight against ISIS in Syria is inching closer to victory, the worst may not be over. Al Qaeda-linked militants continue to make significant advances and maintain territorial control some 150 miles west of the de facto caliphate capital of Raqqa, sweeping through the war-shattered country's province of Idlib.

Late last month the extremist faction Hayat Tahrir al Sham (HTS) waged war with more mainstream Ahrar al-Sham, successfully seizing control of most of Idlib and capturing a key border crossing into Turkey. The increasing HTS dominance could see the U.S. dragged further into the conflict, especially given the credible threat Al Qaeda still poses to the U.S. homeland.

HTS was previously known as Syria’s Al Qaeda (AQ) branch al-Nusra Front, and later named Jabhat Fateh al-Sham.

“The United States continues to target Al Qaeda in Idlib governorate and across Syria in order to protect the homeland from terrorist attacks,” Eric Pahon, a spokesperson for the Department of Defense, told Fox News. “U.S. and Turkish officials have discussed their mutual concerns with the situation developing in Idlib, and we continue to monitor the situation to ensure the U.S. homeland, Turkey and Europe and the broader region are not threatened by terrorist attacks emanating from Idlib.”

Michael Ratney, the U.S. State Department’s Syria envoy, has called the HTS triumph one of the “biggest tragedies” in the ongoing war. Another State Department official, who requested anonymity, told Fox News that the HTS takeover has indeed become a cause for concern.

“The core of HTS is Al Qaeda’s affiliate in Syria,” the official said, noting that despite name changes, the terrorist group remains committed to “eventually conducting attacks against the United States.” The official stressed that the U.S. is “committed to taking any and all steps toward countering al-Nusra,” but declined to elaborate on whether plans were in motion for any military developments.

The Syrian civil war was ignited in spring of 2011 following peaceful protests against the regime of President Bashar Assad, but it quickly spawned into one of world history’s worst civil wars which has subsequently claimed the lives of an estimated 500,000 people. Idlib remains the last major government opposition stronghold, and its population has burgeoned from a pre-war estimate of less than 1.5 million to more than 2 million, as hundreds of thousands of displaced persons from other former rebel bastions such as Homs and Aleppo have flocked to the decimated and overcrowded governorate.

Youssef Sadaki, a media coordinator for the Damascus Media Center, told Fox News that HTS forces slowly started to take over economic matters, communications and policy-making in the area several months prior to their mass offensive – and given the lack of support from the international community – the moderate factions committed to the principles of a dictator and extremist-free Syria were gradually weakened.

“A terrorist militia now controls the capabilities of millions and threatens Kurdish areas as well. HTS controls every point, and the troops coming in from Arsal on the Lebanese border are only giving more power to HTS. That means no group on the ground has the strength to fight them,” Sadaki said. “This is a real war and this will affect the U.S interests outside the Middle East. These non-state actors spread like fungus everywhere.”

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Scores of Syrian fighters, families, refugees and jihadists have also been bused into the devastated area from the Lebanese border town of Arsal over the past ten days. This comes as part of a deal in which the Lebanese militia Hezbollah – which recently captured Arsal – allowed them safe passage to leave in exchange for the release of Hezbollah prisoners being held by the Al Qaeda-connected jihadists in Idlib.

However, some contend that the recent arrivals has only deepened the humanitarian and terrorism crisis.

“HTS doesn’t even try to hide its Al Qaeda relations; these are some bad people,” noted Bassam Barabandi, a former Syrian diplomat and political adviser to the official opposition negotiations team at the Syrian High Negotiations Committee. He pointed out that the dire conditions – complete with hundreds of thousands stranded and homeless on the Turkish border – has become the optimum breeding ground for HTS/Al Qaeda to recruit.

And one local, Asim Zidan, stressed that circumstances on the ground have been deteriorating by the day.

“HTS is trying to control everything in civilian life. They pretended to be part of our revolution to win support from civilians and make their first steps toward governance. They have started to build courts, there are Islamic police in many areas now,” he added. “We are trying to educate the youth not to join them. We are working to mobilize civilians to demonstrate against them; we must not let them control.”