US military officials have denied reports that the latest batch of American-trained Syrian rebels had defected to an al-Qaeda affiliate.
The Pentagon is under intense scrutiny over its "train-and-equip" program in which moderate Syrian rebels are given instruction and weapons to fight Islamic State jihadists in the war-torn country.
Several news and social media reports said the newest batch of about 70 rebels to graduate from the program – who were sent back to Syria last weekend – had defected to al-Qaeda's Syria affiliate, the al-Nusra Front, then handed over their weapons.
"It's patently false that there have been defections or weapons turned over... we believe the claims to those effect to be untrue," Pentagon spokesman Captain Jeff Davis told reporters on Wednesday (local time).
The program to train and arm moderate Syrian rebels to fight IS jihadists originally aimed to ready around 5400 vetted fighters a year for three years.
But the pricey endeavour, which cost US$500 million, got off to a disastrous start.
The first graduates, who made up a group of 54 fighters, were attacked by al-Nusra in July and the Pentagon isn't sure what happened to them all. At least one was killed.
A second setback would cause further embarrassment for America, but Davis stressed the new batch of fighters was not lost.
"We do have contact, we have communications with members of the group and our understanding from being in touch ... is that all are present and accounted for, as are their weapons," Davis said.
The program has also been hamstrung by a lack of suitable recruits who are able to pass the US screening process.
Last week, before the insertion of the new fighters, the US general overseeing efforts against IS drew disbelief from senior lawmakers when he told them only "four or five" US-trained rebels were on the ground fighting in Syria.