[Read about a Times reporter’s tour of the ISIS prisoner camps in Syria]

There have been reports that the Kurdish group was planning to withdraw its forces from front-line positions fighting the Islamic State, but there was no sign so far that had happened.

A report by the Syrian Observatory said the S.D.F. leadership was discussing the prisoners’ release because the home countries of many of them had refused to take them back. The observatory, a London-based group with a network of citizen monitors throughout Syria whose work is widely considered credible, said the prisoners come from 31 countries in addition to Syria, and their family members from 41 countries.

The S.D.F. was also concerned that it would need all of its fighters to defend against a possible Turkish military invasion, the report said — a prospect made more likely by a United States withdrawal.

The Syrian Democratic Forces are predominantly made up of Kurdish fighters from the Y.P.G., the People’s Protection Units, but under American tutelage the group has signed up many Arab fighters opposed to the Islamic State; Arabs now make up about 40 percent of the force, which has up to 75,000 fighters. The group has been trained, advised, financed and supplied by the United States, which has 2,000 troops, mostly Special Operations forces, in Syria allied with the S.D.F.

With American support, especially from airstrikes, since 2016 the Kurds have pushed the Islamic State out of most of the territory it held in eastern and northern Syria, reducing the extremists to a pocket of about 20 square miles on the Iraqi border, near the town of Hajin, far from any cities. Fighting continues in that area, although the S.D.F. claimed last week to have ousted the Islamic State from Hajin.

Turkey has vowed to attack the S.D.F. because it considers the Y.P.G. as a front for the outlawed Peoples Workers Party, or P.K.K., in Turkey, and last week it said a cross-border invasion to attack the Kurds was only days away.

That led to a telephone conversation between Mr. Trump and President Recep Tayyip Erdogan of Turkey last Friday, after which the United States quietly began making preparations for the troop withdrawal.