ISIS fighters and other terrorists comprising the more than 10,000 Islamic militants jailed in northeast Syria could launch a mass prison break as U.S. troops withdraw from the region in response to Turkey's impending incursion, Syrian Kurdish fighters warned Monday.

The imminent cross-border military operation pitting Turkey against the Syrian Kurds -- a group Turkish President Recep Tayyip Erdogan views as terrorists amid a bloody Kurdish insurgency plaguing his country -- is likely to further destabilize an already volatile area and has already led Kurds there to accuse the U.S. of abandoning them. U.S. troops have fought alongside the Syrian Kurdish fighters for years in the successful effort to topple ISIS' so-called caliphate that put the terror group in control of huge swaths of Syria and Iraq. But now, the 1,000 or so American troops left in the northern part of the country will begin pulling back as Turkish forces push ahead.

“The American forces did not abide by their commitments and withdrew their forces along the border with Turkey,” the Kurdish-led Syrian Democratic Forces said in a Monday statement, warning Turkey's offensive would have a "huge negative effect" on the continuing campaign to wipe out ISIS.

Mustafa Bali, the SDF spokesman, said in a tweet that his group was not expecting the U.S. to protect northeastern Syria. But he also slammed the troop exodus for essentially removing a major roadblock that had managed to keep Turkey's ambitions in the region in check.

“People here are owed an explanation regarding the security mechanism deal and destruction of fortifications," Bali tweeted.

Turkish forces artillery pieces are seen on their new positions near the border with Syria in Sanliurfa province on Sunday. (AP/DHA)

TURKEY ANNOUNCES INCURSION OF NORTHEAST SYRIA, US-BACKED KURDS HAVE VOWED 'ALL-OUT' WAR

And while news of the U.S. pullout stunned Syrian Kurds, the announcement also "completely blindsided" top brass at the Pentagon, U.S. officials told Fox News.

Aside from the existential threat to the Kurdish fighters posed by Turkey, Syrian Kurdish forces are also warning that ISIS sleeper cells are actively plotting to free about 12,000 militants currently detained by the Kurds and may take advantage of the Turkey-triggered turmoil to aid their plans.

Those in custody include about 2,500 foreign fighters from Europe and elsewhere whose native countries have been reluctant to take them back — and about 10,000 other captured fighters from Syria and Iraq.

The U.S., which has helped the Syrian Kurds keep watch over the prisoners, said in a statement late Sunday that Turkey will now take custody of the thousands of militants.

The statement added U.S. troops “will not support or be involved in the [Turkish] operation” and “will no longer be in the immediate area” of the incursion in northern Syria. One senior U.S. official that spoke to the Associated Press on condition of anonymity said American troops could leave the country entirely if widespread fighting breaks out between Turkish and Kurdish forces.

In this image provided by Hawar News Agency, U.S. military vehicles travel down a main road in northeast Syria on Monday. (AP/ANHA)

TRUMP'S DECISION TO MAKE WAY FOR TURKEY IN SYRIA SEEN BY CRITICS AS A KURDS BETRAYAL

In an agreement between Ankara and Washington, joint U.S and Turkish aerial and ground patrols had started in a security zone that covers over 78 miles along the Turkey-Syria border between the towns of Tal Abyad and Ras al-Ayn. The SDF had removed fortification from the areas -- considered a threat by Turkey -- and withdrawn with heavy weapons.

But Turkey and the U.S. have disagreed over the depth of the zone, with Ankara seeking to also have its troops monitor a stretch of territory between 19 to 25 miles deep.

The Kurdish-led fighters have been the main U.S.-backed force in Syria in the fight against ISIS and, in March, the group captured the last sliver of land held by the extremists, marking the end of the ISIS caliphate that was declared in 2014 by the group's fugitive leader, Abu Bakr al-Baghdadi.

“We will not hesitate for a moment in defending our people” against Turkish troops, the Syrian Kurdish force said, adding 11,000 fighters have already been killed during the war against ISIS in Syria.

The Kurdish fighters also control the al-Hol refugee camp, home to more than 70,000 people, including at least 9,000 foreigners -- mostly wives and children of ISIS fighters.

The White House’s announcement Sunday to withdraw came a day after Erdogan offered the strongest warning yet that a unilateral military operation into northeastern Syria was on the horizon after recent moves by the Turkish military to dispatch units and defense equipment to its side of the border.

“We have given all kinds of warning regarding the [area] east of the Euphrates to the relevant parties. We have acted with enough patience,” Erdogan said.

The Syrian Kurdish Hawar news agency and the Britain-based Syrian Observatory for Human Rights say American troops started evacuating positions near Ras al-Ayn and Tal Abyad on Monday. A video posted by a Kurdish news agency showed a convoy of American armored vehicles apparently heading away from the border area.

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Turkish Foreign Minister Mevlut Cavusoglu tweeted that Ankara is determined to ensure the survival and security of Turkey “by clearing the region from terrorists.

“We will contribute to peace, peace and stability in Syria,” he said.

Fox News’ Lucas Tomlinson and the Associated Press contributed to this report.