Little is known about the American said to be captured alongside Mr. Clark, beyond his age, 35. The Syrian Democratic Forces did not release where he is from in the United States.
A similarly spelled name — Zaid Abdul-Hamid — appears in a cache of ISIS registration forms indicating that he provided a reference for a recruit from Trinidad and Tobago when that recruit joined the group in 2014. The same form indicates that Mr. Abdul-Hamid was in Raqqa, Syria, at the time that he acted as the reference.
Col. Scott Rawlinson, a spokesman for the American-led military coalition in Baghdad, said in an email that the coalition was “aware of open source reports of reportedly American citizens currently in custody believed to be fighting for ISIS.”
“However, we are unable to confirm this information at this time,” he added. “The incident is under investigation.”
Last month, President Trump’s announcement that the United States would withdraw its forces stationed in Syria prompted the resignation of his defense secretary and accelerated the resignation of the White House’s senior envoy to the fight against ISIS. Both Defense Secretary Jim Mattis and the senior presidential envoy, Brett McGurk, said that the unexpected withdrawal amounted to abandoning America’s allies in the region, especially the Syrian Democratic Forces.
In recent days, White House officials have softened Mr. Trump’s stance, saying that the withdrawal will not happen right away and that it will be carried out in a coordinated manner. On Sunday, the president’s national security adviser, John Bolton, appeared to reverse course, telling reporters in Jerusalem that the pullout was conditional on defeating the last remnants of ISIS and on Turkey’s ensuring the safety of America’s Kurdish allies.
According to three separate reports, including an assessment by the Pentagon’s inspector general, ISIS still maintains a force of 20,000 to 30,000 members in Iraq and Syria.
The last time the group was declared defeated in 2010, the C.I.A. estimated that it was down to just 700 fighters.