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(CNN) —  

The number of foreign fighters being held by the US-backed Syria Democratic Forces has surpassed 800 for the first time, according to the Pentagon.

There are more than “800 foreign terrorist fighters” in SDF custody from “over 40 countries,” Department of Defense spokesman Cmdr. Sean Robertson told CNN.

Officials have expressed concerns about the fate of the detainees following President Donald Trump’s decision to withdraw US troops from Syria.

While the Kurdish-led Syrian Democratic Forces have been able to operate detention facilities, there are concerns as to what would become of the detainees being held there should Turkey attack the Kurds following a US withdrawal.

The US-backed and Kurdish-led SDF have captured multiple foreign ISIS fighters in recent weeks as the US allies drive ISIS from its last remaining towns in Syria along the Euphrates River.

Officials believe that the number of foreigners captured is likely to grow as many of those fighters remaining in ISIS areas are likely to be foreigners unable to attempt to blend in with local populations to escape.

The US-led military coalition has backed the SDF with air and artillery strikes, conducting 645 strikes in Syria from January 13 to 26, destroying ISIS vehicles, buildings, vehicle borne improvised explosive devices, weapons caches, and an unmanned aircraft system.

“I’d say 99.5%-plus of the ISIS-controlled territory has been returned to the Syrians,” acting Secretary of Defense Pat Shanahan told reporters at the Pentagon on Tuesday.

“Within a couple weeks, it’ll be 100%,” he added.

Senior US officials have encouraged countries to repatriate their citizens in detention in Syria in order to ease the burden on the SDF’s detention facilities but has had limited success in doing so.

Many countries are reluctant to do so because of the difficulty of prosecuting suspected ISIS members based on evidence collected on the battlefield.

Earlier this month, the US Department of Justice charged an American who was captured recently in Syria by the SDF and transferred to Texas, where he was indicted by a federal grand jury on charges of attempting to help ISIS.

The US military has also facilitated the return of foreign terrorist fighters from Syria to Kazakhstan and Macedonia.

US officials say additional fighters have been repatriated by several countries that have chosen to not make those repatriations public.