The Pentagon has said that a group of US-trained Syrian fighters has handed over ammunition and equipment to al-Nusra Front, al-Qaeda's affiliate in the country, purportedly in exchange for safe passage.
The acknowledgement contrasted with earlier denials by the US defence department of reports that some fighters had either defected or handed over gear.
"Unfortunately, we learned late today that the NSF (New Syrian Forces) unit now says it did, in fact, provide six pickup trucks and a portion of their ammunition to a suspected al-Nusra Front [group]," Pentagon spokesman Captain Jeff Davis said on Friday.
Colonel Patrick Ryder, a spokesman for Central Command, which is overseeing efforts against the Islamic State of Iraq and the Levant (ISIL), said the fighters had handed over the gear in exchange for safe passage in the Nusra operating area.
"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," Ryder said.
Ryder added that the pickup vehicles and ammunition represented about 25 percent of the equipment issued to the group by the US-led coalition.
"We are using all means at our disposal to look into what exactly happened and determine the appropriate response," Ryder said.
A defence official told the AFP news agency that according to the NSF, there had not been any defections, but he stressed: "We only know what they have told us."
The development is another embarrassing setback for the US effort to "train and equip" Syrian fighters to take on ISIL in Syria.
The $500m programme originally aimed to ready about 5,400 vetted fighters a year for three years but problems finding suitable candidates have seen only a tiny fraction getting trained.
The first graduates, who made up a group of 54 fighters, were attacked by Nusra in July and the Pentagon is still unsure what happened to them all, with at least one killed.
The second group, consisting of about 70 fighters, were sent back to Syria last weekend and reports began circulating on Twitter soon after that they had either defected or handed over equipment.
Last week, before the insertion of the new fighters, the US general overseeing efforts against ISIL drew gasps of disbelief from senior politicians when he told them only "four or five" US-trained rebels were on the ground fighting in Syria.
Unwilling to commit US ground troops in the region, the Obama administration in January launched the "train and equip" mission for Syrian opposition fighters as part of a broader push to work with locals there and in Iraq.
The programme has faltered, with the Pentagon saying many would-be fighters had failed the strict screening process.