WASHINGTON — A Syrian rebel commander handed over to an al-Qaida affiliate trucks and ammunition provided by the U.S.-led coalition, in exchange for safe passage to a Syrian town, according to details released by the U.S. military.

The military's account has raised more questions about the troubled U.S. program to back moderate rebels and is reinforcing pressure to make changes. The program has seen failures in both of the classes of recruits that graduated and returned to Syria to fight the Islamic State.

Air Force Col. Pat Ryder, U.S. Central Command spokesman, said Monday that military leaders are continuing to review the program and dig into the details of the equipment loss last week. He said the military is doing everything it can to screen the recruits and to vet the rebel units and commanders.

"We will look at what we can do to prevent such a situation in the future," Ryder said, "but given the complexity of the battlefield it is not possible to eliminate all risk.

"The moderate and vetted opposition continues to face threats on multiple fronts, including from various extremist groups. It's important to be clear-eyed about the conditions in which these forces operate, the groups working against them, and the need to overcome the challenges they have encountered and will continue to face," he said.

After several days of review, the U.S. on Monday released more details about the incident after first saying last week that reports about a rebel commander handing over equipment to the Nusra Front were false. On Friday, Central Command said that in fact the commander had handed over about a quarter of the equipment the coalition had given his unit.

Ryder said that 30 of the fighters trained by the U.S. returned to a vetted Syrian unit, but the commander had not gone through the training. The U.S. works with vetted Syrian rebel group commanders and leaders to identify volunteers who will agree to fight the Islamic State, not the Syrian government. The volunteers are vetted and once they go through the training, they return to their unit. The coalition provides equipment, weapons and air support to the vetted groups

Ryder said the commander told U.S. officials that he was contacted on Sept. 20 by an intermediary linked to the Nusra Front, which is an al-Qaida affiliate in Syria. The Nusra fighters told him he had to turn over equipment to them in exchange for safe passage of his troops from one town to another, the commander told US officials.

Officials said that in the next two days he gave up six trucks and some ammunition.

U.S. officials were unaware of the problem for several days, even as media reports began to surface about the possible transfer of equipment to the militants. The commander didn't tell the U.S. what had happened until five days after he was initially contacted by the Nusra Front.

The gaps in time raise questions about why the commander didn't simply report the threat to the U.S. or coalition leaders and why the newly trained members of the unit weren't prepared to fight the militants rather than give up the equipment.

The U.S. has had repeated problems with the training, triggering an ongoing overhaul of the effort, including suggestions that the newly trained fighters operate as the New Syrian Forces, or NSF, alongside Syrian Kurds, Sunni Arab and other anti-Islamic State forces.

The first batch of about 54 trainees that returned to the battlefield has largely disbanded. Of the 54, one was killed; one is being held captive; nine are back in the fight; 11 are available but not in Syria; 14 returned to Syria but quit the U.S. program; and 18 are unaccounted for.

The third and fourth classes — totaling about 50 fighters — are still at the training sites, but could be sent back to rebel units in Syria soon.