Washington (CNN) Fears are growing among American officials that thousands of ISIS fighters may escape from prisons in Syria as the Kurdish personnel guarding them gear up for a fight with Turkey, which launched a military offensive in northeastern Syria on Wednesday.

Turkey's assault has already had a "detrimental effect" on American counter-ISIS operations, which have "effectively stopped," a senior US defense official told CNN on Wednesday.

The Turkish offensive, the official said, "has challenged our ability to build local security forces, conduct stabilization operations and the Syrian Democratic Forces' [ability] to guard over 11,000 dangerous ISIS fighters."

When asked Wednesday about the threat of ISIS prisoners escaping, President Donald Trump claimed that some of the most dangerous ISIS prisoners had been moved, "putting them in other areas where it's secure."

He dismissed the overall threat, replying, "Well, they're going to be escaping to Europe."

US officials have long warned of the vulnerability of the "pop-up prisons" housing some 11,000 to 12,000 ISIS fighters captured on the battlefield, 2,000 of whom are foreigners not from Iraq or Syria. Despite Trump's assertion, only several hundred of the prisoners are believed to be from Europe.

Those same officials emphasize that the security of the facilities is very much in question and a "huge concern" as Syrian Democratic Forces personnel head to the front lines.

"SDF will focus on the fight. The fear is that they abandon post," said a congressional aide who attended a classified briefing given Tuesday by national security officials on the detention facilities. "It's a really bad situation."

Hundreds of ISIS prisoners continue to be held in makeshift prisons close to the Turkish border, according to US defense officials.

SDF fighters leaving their prison posts.

Some SDF fighters had already left their posts at various prisons before the Turkish offensive began and had headed north, two US officials told CNN. As the operation got underway, the SDF suspended their counter-ISIS operations to focus on the Turkish offensive, another official said.

A US defense official told CNN on Wednesday that while the SDF had reduced the number of personnel guarding the prisons, the US military believes that all of the prisons continue to be guarded, albeit by fewer SDF troops.

Later on Wednesday the SDF said a prison holding ISIS detainees was hit in a Turkish airstrike.

In a Twitter post from the verified SDF account, the group said "One of the prisons that ISIS detainees held in was struck by Turkish airstrike. Turkey is aiming to undermine all successful efforts and achievements that we gained during our fighting against #ISIS."

Trump's decision to pull US troops from the area, thereby giving Turkish President Recep Tayyip Erdogan a de facto go-ahead to proceed with his long-planned move against Kurdish fighters who make up part of the Syrian Democratic Forces and had fought against ISIS with the US has provoked a backlash from Republican allies.

It could also potentially reverse one of Trump's main foreign policy achievements, the elimination of ISIS's so-called caliphate, which the President had heralded in March. Analysts warn that should the ISIS prisoners escape it could be a major step toward the militants regrouping and challenging for territory again.

Two weeks ago, the State Department's counterterrorism coordinator, Ambassador Nathan Sales, warned that "destabilizing events" could "trigger the release of these folks."

"We've seen several attempted prison breaks over the last few months," Sales said. "None have been successful to date, but we don't want to hope that our luck holds. That's not an effective strategy."

Trump has called for prisoners to be repatriated

Sales said more funding and support were needed to reinforce the often-overcrowded facilities but argued the only long-term solution was to repatriate the prisoners, something Trump has repeatedly called for.

"We already did not have professional jails or professional prisons to keep those prisoners in," SDF spokesman Mustafa Bali said. "the Turkish invasion to our region is going to leave a huge space, because we are forced to pull out some of our troops from the prisons and from the [displaced people] camps to the border to protect our people."

On Wednesday afternoon, Bali tweeted that Turkey had shelled an area near a prison holding ISIS prisoners.

The majority of the ad hoc prisons -- often schools or municipal buildings -- lie beyond the 19-mile so-called "safe zone" that Turkey wants to establish along 300 miles on Syria's side of the border. But two US officials say less than half of the prisons are inside, where the fighting is expected to be fiercest.

"These terrorists would join the thousands that are already on the battlefield who are currently conducting operations and planning for prison breaks," Democratic Sen. Jeanne Shaheen of New Hampshire, who has visited the Syrian facilities, said in a statement. "President Erdogan, despite his claims, does not have the support of the international community for this operation and he refuses to assure the US that he will guard ISIS detention facilities in the area and prevent ISIS from once again gaining a foothold in the region."

Turkey has shown little interest in the security of the prisons and the detainees as it prepared to launch its incursion. American officials watching the situation say it's clearly not a top Turkish priority. But in announcing the withdrawal of US troops from the area late Sunday night, Trump said he believes it's now Turkey's job.

"Turkey will now be responsible for all ISIS fighters in the area captured over the past two years in the wake of the defeat of the territorial 'Caliphate' by the United States," the White House said in a statement.