Concerns over a resurgence of the Islamic State in Syria remain heightened following the fall of the Bashar al-Assad regime and an increase in attacks targeting U.S.-aligned Syrian Democratic Forces (SDF).

President-elect Donald Trump may well face another round against the extremist group as the SDF faces a reality in which it may have to divide its focus between ISIS and threats levied at it by Turkey.

The SDF said five of its soldiers were killed Saturday in attacks by Turkish-backed forces in northern Syria, reported Reuters. 

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The attacks came following an apparent collapse in a cease-fire agreement brokered by the Biden administration as the U.S. and the SDF ramp up efforts to counter ISIS. 

National security advisor Jake Sullivan on Sunday told CNN that his "single biggest concern" is the return of ISIS, which was deemed "defeated" in 2019. 

"ISIS loves vacuums," he said in reference to the extremist group’s use of power struggles in places like North Africa to gain footholds. "What we see in Syria right now are areas that are basically ungoverned because of the fall of the Assad regime. 

"Our goal is to ensure that we support the SDF — the Kurds — and that we keep ISIS in check," he added.

The U.S. has long had to balance its campaign against ISIS in Syria — which it is fighting with the help of the Kurdish coalition forces, despite Turkey deeming the SDF as akin to the terrorist network the Kurdistan Workers Party (PKK) — with Washington's partnership alongside Ankara as a NATO ally. 

"The SDF and the Assad regime were the primary opponents of ISIS," Bill Roggio, senior fellow at the Foundation for Defense of Democracies and founding editor of "The Long War Journal," told Fox News Digital. "With the former gone and the latter under pressure from Turkish proxies, concerns about the expansion of ISIS are warranted."

"Turkey wants to destroy the SDF," Roggio confirmed. "Turkey has the ideal opportunity to destroy the SDF, and it will take advantage of this unique situation. I expect attack[s] against the SDF to increase."

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The Biden administration has already taken steps to ramp up its campaign against ISIS, hitting more than 75 sites in a significant strike earlier this month on known "ISIS leaders, operatives and camps," U.S. Central Command (CENTCOM) confirmed.

The operation coincided with the fall of Damascus on Dec. 8 following a sweeping takeover of Aleppo, Hama and Homs by Hay’at Tahrir al-Sham (HTS), which was aided by the Turkey-backed Syrian National Army (SNA).

In addition, CENTCOM on Thursday killed ISIS leader Abu Yusif aka Mahmud using a precision airstrike in eastern Syria — an area where, according to Syrian news outlets, ISIS has been able to seize weapons depots belonging to the former Syrian military under the Assad regime amid the "chaos."

SDF forces in an attempt to clamp down on ISIS uprisings captured 18 ISIS terrorists and suspected collaborators on Sunday near the city of Raqqa, which was once an ISIS stronghold, according to ANF News.

The campaign was reportedly done "in cooperation with the international coalition forces," but CENTCOM has not yet confirmed whether the U.S. was involved. 

But concern remains high that the SDF could see its operational abilities divided as attacks from the Turkey-backed SNA coalition forces increase — which could spell trouble for the upcoming Trump administration as it looks to prevent another resurgence of ISIS, while balancing U.S. relations with Turkey, which is further expected to exercise outsized influence over the new Syrian government.

"We continue to monitor the situation in Syria," Brian Hughes, Trump-Vance Transition spokesperson said in response to questions from Fox News Digital. "President Trump is committed to diminishing threats to peace and stability in the Middle East and to protecting Americans here at home."