US imported arms and equipment originally given to the few chosen Syrian fighters, are now in the possession of the Nusra Front, Pentagon reports.
The Pentagon has said the small group of US-trained Syrian fighters, has handed over American equipment and ammunition to the rebel group in exchange for safe passage.
"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.
US Defence Department had previously denied reports that some US-trained fighters had either defected or handed over arms to rebel groups inside of Syria.
Colonel Patrick Ryder, a spokesman for Central Command, which is overseeing efforts against ISIS, confirmed the incident.
The Syrian rebel fighters, including the Nusra Front, have been battling the brutality of the Syrian regime forces and the aggressively rising ISIS insurgency for four years.
"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.
An existing US-led coalition is currently battling ISIS via air strikes in both Syria and Iraq, ideologically the US and the Syrian opposition -who is fighting against the Syrian regime which has killed over 200,000 civilians- are not currently on the same side.
The ammunition, arms, pickup vehicles handed over to Nusra Front represented about 25 percent of the total American equipment issued, Ryder confirmed
"We are using all means at our disposal to look into what exactly happened and determine the appropriate response," Ryder said.
With the US focus being the battle against ISIS, the opposition is hoping that the US will add the Syrian regime to the list of targets. Although the US no longer recognises the legitimacy of the Syrian regime leader, Bashar al Assad, the US-led coalition is not aimed at the regime forces, and has been frequently accused by Syrian activists of not training enough fighters to begin with.
"We only know what they have told us." a defence official told the AFP over the alleged defections.
The Syrian conflict which has been ongoing since March 2011, has killed over 240,000 people and displaced millions both internally and externally.
The US train and equip programme has been progressing slowly and is yet to achieve any of its initial goals, in spite of the whopping $500m dedicated to its development.
Only a small number of 54 fighters are enrolled in the first batch, they were the only ones deemed ‘moderate enough’ to receive the American training.
They were allegedly attacked by the Nusra Front in July and the Pentagon is still unsure as to what happened to them.
There were 70 fighters in the second batch, and they were sent back to Syria in September, reports have surfaced alleging that they have defected.
The US military launched the programme in May, aimed at training up to 5,400 fighters a year in Turkey to combat ISIS in Syria.