The startling acknowledgement contrasted with earlier Pentagon denials 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.

Colonel Patrick Ryder, a spokesman for Central Command (CENTCOM), which is overseeing efforts against the ISIS group in Iraq and Syria, said the fighters had handed over the gear in exchange for safe passage in the Al Nusra operating area.

He added that the pickup vehicles and ammunition represented about 25 per cent of the equipment issued to the group by the US-led coalition.

The development is another embarrassing setback for the US effort to "train and equip" wgat it called “moderate Syrian rebels” to fight ISIS terrorists in Syria.

The $500 million program originally aimed to ready around 5,400 vetted fighters a year for three years but problems finding suitable candidates have seen only a fraction getting trained.

The first graduates, who made up a group of 54 fighters, were attacked by Al Nusra in July and the Pentagon is not sure what happened to them all. At least one was killed.

The second group, consisting of about 70 rebels, were sent back to Syria last weekend and reports began circulating on Twitter soon after that they had either defected or handed over equipment.