A Free Syrian Army fighter takes a position as he aims his weapon in Aleppo in September 2013. (MUZAFFAR SALMAN/REUTERS)

The debut of a new U.S.-trained force in Syria suffered another setback on Tuesday when five American-backed rebels were apparently abducted by al-Qaeda’s Syrian affiliate, the latest of several attacks on that group in the past week.

A U.S. defense official, speaking on the condition of anonymity to discuss events in Syria, said it appeared that five Syrian fighters allied with the United States had been taken prisoner in the past few days by another armed group, probably Jabhat al-Nusra. Military officials said the Syrians appeared to have been abducted after they set out from their compound near the Syrian town of Azaz.

But with no U.S. troops on the ground to track the situation more closely, the official cautioned that the details of what took place remained murky. “It is a dynamic situation on the ground there,” he said.

News of the abduction comes several days after the leader of Division 30, the Syrian opposition unit from which the United States has pulled cadets for its new training program, and other members of his unit were captured by the Islamist group. At the time, U.S. officials stressed that none of those abducted had gone through recent training in Turkey.

The first cohort of 54 Syrians to graduate from the training program, which aims to build up a force to fight the Islamic State, a separate Islamist group from Jabhat al-Nusra, crossed back into Syria in July. Trainers have struggled to fill spots in the program, partly because of a requirement that trainees pledge to fight only the Islamic State and not the Syrian government.

On Friday, U.S. warplanes dropped bombs on Jabhat al-Nusra fighters who had attacked members of what U.S. officials are calling the New Syrian Force.

The defense official declined to say what additional actions the Pentagon would take to assist the abducted Syrians. The Obama administration has been reluctant to send U.S. forces into Syria and has done so only a handful of times since the conflict began there in 2011. Hostage rescue operations are especially risky. In 2014, U.S. commandos staged an unsuccessful operation to rescue American hostages in Syria.

“As a practical matter, air support is one thing,” the official said. “Hostage rescue is a different thing.”