Colonel Patrick Ryder, U.S. Central Command spokesman, said U.S. military has mistakenly bombed fighters it has equipped and trained to fight ISIS in northern Syria on late May.

Col. Ryder said that U.S. fighter jets conducted three airstrikes against ISIS position in and around the northern city if Ma’ra (nearly 25 km to the north of Aleppo) to help US-backed fighters against ISIS.

However, leaders from the FSA-affiliated Mutasim Brigade confirmed that at least 10 members of the were killed in the airstrikes.

The incident prompted an internal investigation and raised questions about the efforts being exerted to build capable moderate forces to confront the jihadi group.

The U.S. military is supporting a Kurdish-led force to battle ISIS in northern Syria.