The United States military says Syrian rebels they have trained have given vehicles and ammunition to the al-Qaeda-linked Nusra Front.
Six pick-up trucks and about one-quarter of their issued equipment was handed over apparently in exchange for safe passage.
The US has trained rebels to fight militants from the group calling itself Islamic State (ISIL).
A statement issued by US Central Command (Centcom) said the report, if accurate, is concerning and a violation of Syria train-and-equip programme guidelines.
Centcom overseas military operations in the Middle East. It was told of the equipment surrender on Friday evening, according to the statement. Earlier the same day, a Centcom spokesman had said all weapons issued to the rebels remained under their control.