The Pentagon had previously indicated that photos of the military items paraded by ISIS on the Internet this weekend
were likely American and lost during recent operations in Afghanistan.
On Tuesday, Brig.-Gen. Charles Cleveland, chief spokesman for the US-led anti-ISIS coalition, said US troops came under "effective enemy fire" in eastern Afghanistan on July 25.
This forced them to abandon their position and led to some of their equipment being captured and displayed in the ISIS photos distributed by ISIS, according to Cleveland.
The photos depict ammunition, fabric packs and possibly a shoulder-fired rocket launcher of the type used by US forces.
US troops, including Special Operations Forces, had been in combat against ISIS in their stronghold in Afghanistan's Nangarhar Province for several days when the incident occurred in July.
Wounded troops had already been moved from the area at the time the equipment was left.
But as a result of the incoming fire, the collection point "was moved to a safer location," in the course of which "some equipment was left behind," Cleveland said in an emailed statement.
"For understandable reasons, the lives of the soldiers were not put at risk to recover the equipment," he continued. "Neither US positions or personnel were overrun."
It is not clear why the US was not able to repel the incoming fire.
For many years, the Taliban have scoured the battlefield for US military gear and weapons, so it's not known for certain what US military unit all the items displayed came from. But a coalition official said that some of the equipment shown "is used" by Special Operations.
The fighting in Nangahar against ISIS has resulted in injuries to several US troops.
Three US soldiers suffered minor wounds in the last few days when an improvised explosive device struck their convoy while they were on patrol near Jalalabad. A NATO Resolute Support mission spokesperson told CNN that the soldiers suffered minor injuries.
In the same group of recently released photos, ISIS displayed the military identification card of US Army specialist Ryan Jay Larson, claiming they had captured him. US commanders were able to quickly determine Ryan had lost his card during the firefight on July 25 but was with his unit at all times.