The mother of James W. Foley, the first American hostage beheaded by the Islamic State, expressed concern Thursday that Turkey’s assault on Syrian Kurds, who have been guarding thousands of ISIS detainees in Syria, would lead to prisoner escapes, resurrection of the extremist group and dashed hopes for punishment of the killers of her son and others.

“This is an international security threat,” the mother, Diane Foley, said in a telephone interview, giving her assessment of the consequences of President Trump’s decision to pull back American forces in northern Syria. That decision allowed Turkey to attack the Syrian Kurdish militia known as the Syrian Democratic Forces — America’s principal ally in battling the Islamic State.

The Turkish invasion, which began Wednesday, called into question the Kurdish militia’s ability to continue securely holding 11,000 captured ISIS fighters in Syria. They include many who had slipped into the country via the porous border with Turkey when the Islamic State commanded a large swath of Syria and Iraq more than five years ago as part of its now-defunct caliphate.

“I can understand President Trump saying, ‘We can’t take care of everything,’” Mrs. Foley said. Nonetheless, she said, “at least the Kurds were making sure the prisoners were secure.”