A group of US-trained Syrian rebels has handed over their vehicles and ammunition to fighters linked to al-Qaeda, the US military has admitted.
It said one rebel unit had surrendered six pick-up trucks and ammunition to the al-Nusra Front this week - apparently to gain safe passage.
Congress has approved $500m (£323m) to train and equip about 5,000 rebels to fight against Islamic State militants.
But the first 54 graduates were routed by al-Nusra Front, the military said.
Gen Lloyd Austin told US lawmakers last week that only "four or five" US-trained rebels were still fighting.
"Unfortunately, we learned late today that the NSF (New Syrian Forces) unit now says it did in fact provide six pick-up trucks and a portion of their ammunition to a suspected al-Nusra Front (group)," Pentagon spokesman Cpt Jeff Davis said on Friday.
Meanwhile, Col Patrick Ryder, a spokesman for US Central Command (Centcom), said this happened on 21-22 September.
He added that the surrendered vehicles and ammunition amounted to roughly 25% of the equipment issued to the unit.
"If accurate, the report of NSF members providing equipment to al-Nusra Front is very concerning and a violation of Syria train-and-equip programme guidelines," Col Ryder said.
The unit was part of some 70 rebel fighters who participated in the second US training course.
The train-and-equip programme is at an early stage, but this is just the latest in a series of setbacks, the BBC's Laura Bicker in Washington says.