As of July, the US had only trained 60 Syrian fighters to take on ISIS.
The Obama administration was hoping to build a force to neutralize ISIS.
ISIS has 20,000 to 30,000 fighters.
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But several of the US-trained rebels were captured when they entered Syria in July.
The remaining US-trained fighters are now refusing to fight, saying they were misled by the Obama administration.
Syrian fighters trained by the United States are now refusing to fight.
In the past few days, five of the US backed recruits have been detained by the al-Qaeda-affiliated al-Nusra Front. A sixth recruit has reportedly been killed.
The US-backed group, Division 30, is accusing the Pentagon of misrepresenting its mission.
The fighters say they signed up to battle the Islamic State of Iraq and the Levant (ISIL), not the al-Nusra Front.
The group is also opposed to US air strikes carried out on al-Nusra Front fighters in recent days.
In a statement, the Pentagon denied it distorted its battle plan.
The 54-strong rebel unit, trained and equipped by the Pentagon, was inserted into Aleppo province in mid-July as part of US plans to forge a moderate force for the campaign against the ISIL.
But despite al-Nusra’s fierce hostility of ISIL, fighters from the Division 30 soon came under attack by the al-Qaeda loyalists, who believe that the US-backed battalion would end up battling them. Al-Nusra is considered a terrorist organisation by the US and other Western countries.