For years, U.S.-backed Kurdish fighters in Syria paid with their lives to defeat the Islamic State militant group.

In the wake of President Donald Trump's announcement that U.S. forces were pulling back from northeast Syria, the remaining fighters worry that their comrades' sacrifices will have been in vain.

"We have more work to do to keep ISIS from coming back and make our accomplishments permanent," Mustafa Bali, the spokesperson for the Kurdish-led Syrian Democratic Forces, tweeted Tuesday. "If America leaves, all will be erased."

The SDF has been a crucial U.S. ally in the war against ISIS and currently controls much of the area close to the border with Turkey. The forces say they have lost 11,000 fighters during the struggle.

In March, the group captured the last sliver of land held by the extremists, marking the end of the so-called caliphate that was declared by ISIS' leader Abu Bakr al-Baghdadi in 2014.

Now they are caught between a U.S. president anxious to deliver on promises to remove America from foreign wars, and a Turkish government that sees Kurdish fighters as terrorists who threaten the integrity of its country.

A truck carrying men, identified as Islamic State group fighters who surrendered to the Syrian Democratic Forces, leaves ISIS' last holdout of Baghouz on Feb. 20. Bulent Kilic / AFP - Getty Images file

On Monday, Trump told senior military leaders that some 50 troops had moved out of northeast Syria, after the White House said Sunday it would not stand in the way of Ankara launching an operation in the region. There are about 1,000 American troops in Syria.

As news of the dramatic shift spread, a top Kurdish general told NBC News that SDF fighters assigned to guard thousands of captured extremists had started to rush to the border ahead of an expected Turkish attack.

Some 12,000 suspected terrorists in detention centers guarded by Kurdish forces are now a “second priority,” according Gen. Mazloum Kobani, commander of the Syrian Democratic Forces. Of the 12,000, some 2,000 are foreign fighters while the others are Iraqis and Syrians, Pentagon officials say.

On Tuesday, the Turkish defense ministry said in a tweet that all preparations for the operation had been completed and that its forces aimed to create a “safe zone” where Syrians can resettle. More than 2.5 million Syrians who fled their country's civil war live in Turkey.

The ministry added that Turkey would not tolerate the establishment of a “terror corridor” on its borders.

Trump denied the United States was cutting and running.

“We may be in the process of leaving Syria, but in no way have we abandoned the Kurds, who are special people and wonderful fighters,” he said in a tweet Tuesday.

However, experts warned that the withdrawal of U.S. troops, and a looming Turkish invasion expected to start any day, would be a gift for the remnants of the terror group, which thrives in the vacuum created by instability and violence.

“The main beneficiary of the withdrawal of the American forces is ISIS,” said Fawaz Gerges, a professor of Middle Eastern politics at the London School of Economics.

“You’re going to see — and I hope I’m wrong — ISIS fighters and sleeping cells coming out and trying to really capitalize on the violence if and when Turkey invades northeastern Syria,” Gerges said. For ISIS, “it’s a godsend.”

In its most recent quarterly annual report on U.S. operations in Syria, released in August, the Defense Department's inspector general said that "ISIS remains a threat in Iraq and Syria."

While the White House says Turkey will now be responsible for all ISIS fighters in the area captured over the past two years, and stresses that the ISIS territorial caliphate had been defeated, experts and officials are also worried about further radicalization within prisons and refugee camps.

Last month, U.S. envoy James Jeffrey said radicalization in the notorious al-Hol camp was “on everybody’s mind." Gerges described it Tuesday as a “hotbed for the resurgence of ISIS ideology.”

A U.S. soldier stands guard during a joint patrol with Turkish troops in the Syrian village of al-Hashisha along the border with Turkey, on Sept. 8. Delil Souleiman / AFP - Getty Images file

Al-Hol is home to more than 70,000 people, including at least 9,000 foreigners, mostly wives and children of extremist fighters.

Karin Von Hippel, director-general of the Royal United Services Institute think tank in London, said if the Kurds withdrew and left ISIS prison camps to be attacked by extremists, the fighters and their families could become the “nucleus of ISIS 2.0 or 3.0.”

“ISIS could just take over the prisons if the Kurds leave,” she said. “I’m sure they’re planning that as we speak.”